Use mypy 1.0 (#15052)

* Update mypy and mypy-zope
* Remove unused ignores

These used to suppress

```
synapse/storage/engines/__init__.py:28: error: "__new__" must return a
class instance (got "NoReturn")  [misc]
```

and

```
synapse/http/matrixfederationclient.py:1270: error: "BaseException" has no attribute "reasons"  [attr-defined]
```

(note that we check `hasattr(e, "reasons")` above)

* Avoid empty body warnings, sometimes by marking methods as abstract

E.g.

```
tests/handlers/test_register.py:58: error: Missing return statement  [empty-body]
tests/handlers/test_register.py:108: error: Missing return statement  [empty-body]
```

* Suppress false positive about `JaegerConfig`

Complaint was

```
synapse/logging/opentracing.py:450: error: Function "Type[Config]" could always be true in boolean context  [truthy-function]
```

* Fix not calling `is_state()`

Oops!

```
tests/rest/client/test_third_party_rules.py:428: error: Function "Callable[[], bool]" could always be true in boolean context  [truthy-function]
```

* Suppress false positives from ParamSpecs

````
synapse/logging/opentracing.py:971: error: Argument 2 to "_custom_sync_async_decorator" has incompatible type "Callable[[Arg(Callable[P, R], 'func'), **P], _GeneratorContextManager[None]]"; expected "Callable[[Callable[P, R], **P], _GeneratorContextManager[None]]"  [arg-type]
synapse/logging/opentracing.py:1017: error: Argument 2 to "_custom_sync_async_decorator" has incompatible type "Callable[[Arg(Callable[P, R], 'func'), **P], _GeneratorContextManager[None]]"; expected "Callable[[Callable[P, R], **P], _GeneratorContextManager[None]]"  [arg-type]
````

* Drive-by improvement to `wrapping_logic` annotation

* Workaround false "unreachable" positives

See https://github.com/Shoobx/mypy-zope/issues/91

```
tests/http/test_proxyagent.py:626: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:762: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:826: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:838: error: Statement is unreachable  [unreachable]
tests/http/test_proxyagent.py:845: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:151: error: Statement is unreachable  [unreachable]
tests/http/federation/test_matrix_federation_agent.py:452: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:60: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:93: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:127: error: Statement is unreachable  [unreachable]
tests/logging/test_remote_handler.py:152: error: Statement is unreachable  [unreachable]
```

* Changelog

* Tweak DBAPI2 Protocol to be accepted by mypy 1.0

Some extra context in:
- https://github.com/matrix-org/python-canonicaljson/pull/57
- https://github.com/python/mypy/issues/6002
- https://mypy.readthedocs.io/en/latest/common_issues.html#covariant-subtyping-of-mutable-protocol-members-is-rejected

* Pull in updated canonicaljson lib

so the protocol check just works

* Improve comments in opentracing

I tried to workaround the ignores but found it too much trouble.

I think the corresponding issue is
https://github.com/python/mypy/issues/12909. The mypy repo has a PR
claiming to fix this (https://github.com/python/mypy/pull/14677) which
might mean this gets resolved soon?

* Better annotation for INTERACTIVE_AUTH_CHECKERS

* Drive-by AUTH_TYPE annotation, to remove an ignore

17 files changed, 209 insertions, 104 deletions

diff --git a/changelog.d/15052.misc b/changelog.d/15052.misc
new file mode 100644
index 0000000000..93ceaeafc9
--- /dev/null
+++ b/changelog.d/15052.misc
@@ -0,0 +1 @@
+Improve type hints.
diff --git a/poetry.lock b/poetry.lock
index e534b30d2b..eb1e3d797b 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -146,14 +146,14 @@ css = ["tinycss2 (>=1.1.0,<1.2)"]
 
 [[package]]
 name = "canonicaljson"
-version = "1.6.4"
+version = "1.6.5"
 description = "Canonical JSON"
 category = "main"
 optional = false
 python-versions = ">=3.7"
 files = [
-    {file = "canonicaljson-1.6.4-py3-none-any.whl", hash = "sha256:55d282853b4245dbcd953fe54c39b91571813d7c44e1dbf66e3c4f97ff134a48"},
-    {file = "canonicaljson-1.6.4.tar.gz", hash = "sha256:6c09b2119511f30eb1126cfcd973a10824e20f1cfd25039cde3d1218dd9c8d8f"},
+    {file = "canonicaljson-1.6.5-py3-none-any.whl", hash = "sha256:806ea6f2cbb7405d20259e1c36dd1214ba5c242fa9165f5bd0bf2081f82c23fb"},
+    {file = "canonicaljson-1.6.5.tar.gz", hash = "sha256:68dfc157b011e07d94bf74b5d4ccc01958584ed942d9dfd5fdd706609e81cd4b"},
 ]
 
 [package.dependencies]
@@ -1146,36 +1146,38 @@ files = [
 
 [[package]]
 name = "mypy"
-version = "0.981"
+version = "1.0.0"
 description = "Optional static typing for Python"
 category = "dev"
 optional = false
 python-versions = ">=3.7"
 files = [
-    {file = "mypy-0.981-cp310-cp310-macosx_10_9_universal2.whl", hash = "sha256:4bc460e43b7785f78862dab78674e62ec3cd523485baecfdf81a555ed29ecfa0"},
-    {file = "mypy-0.981-cp310-cp310-macosx_10_9_x86_64.whl", hash = "sha256:756fad8b263b3ba39e4e204ee53042671b660c36c9017412b43af210ddee7b08"},
-    {file = "mypy-0.981-cp310-cp310-macosx_11_0_arm64.whl", hash = "sha256:a16a0145d6d7d00fbede2da3a3096dcc9ecea091adfa8da48fa6a7b75d35562d"},
-    {file = "mypy-0.981-cp310-cp310-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:ce65f70b14a21fdac84c294cde75e6dbdabbcff22975335e20827b3b94bdbf49"},
-    {file = "mypy-0.981-cp310-cp310-musllinux_1_1_x86_64.whl", hash = "sha256:6e35d764784b42c3e256848fb8ed1d4292c9fc0098413adb28d84974c095b279"},
-    {file = "mypy-0.981-cp310-cp310-win_amd64.whl", hash = "sha256:e53773073c864d5f5cec7f3fc72fbbcef65410cde8cc18d4f7242dea60dac52e"},
-    {file = "mypy-0.981-cp37-cp37m-macosx_10_9_x86_64.whl", hash = "sha256:6ee196b1d10b8b215e835f438e06965d7a480f6fe016eddbc285f13955cca659"},
-    {file = "mypy-0.981-cp37-cp37m-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:8ad21d4c9d3673726cf986ea1d0c9fb66905258709550ddf7944c8f885f208be"},
-    {file = "mypy-0.981-cp37-cp37m-musllinux_1_1_x86_64.whl", hash = "sha256:d1debb09043e1f5ee845fa1e96d180e89115b30e47c5d3ce53bc967bab53f62d"},
-    {file = "mypy-0.981-cp37-cp37m-win_amd64.whl", hash = "sha256:9f362470a3480165c4c6151786b5379351b790d56952005be18bdbdd4c7ce0ae"},
-    {file = "mypy-0.981-cp38-cp38-macosx_10_9_universal2.whl", hash = "sha256:c9e0efb95ed6ca1654951bd5ec2f3fa91b295d78bf6527e026529d4aaa1e0c30"},
-    {file = "mypy-0.981-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:e178eaffc3c5cd211a87965c8c0df6da91ed7d258b5fc72b8e047c3771317ddb"},
-    {file = "mypy-0.981-cp38-cp38-macosx_11_0_arm64.whl", hash = "sha256:06e1eac8d99bd404ed8dd34ca29673c4346e76dd8e612ea507763dccd7e13c7a"},
-    {file = "mypy-0.981-cp38-cp38-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:fa38f82f53e1e7beb45557ff167c177802ba7b387ad017eab1663d567017c8ee"},
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-    {file = "mypy-0.981-cp38-cp38-win_amd64.whl", hash = "sha256:e1acf62a8c4f7c092462c738aa2c2489e275ed386320c10b2e9bff31f6f7e8d6"},
-    {file = "mypy-0.981-cp39-cp39-macosx_10_9_universal2.whl", hash = "sha256:b6ede64e52257931315826fdbfc6ea878d89a965580d1a65638ef77cb551f56d"},
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-    {file = "mypy-0.981.tar.gz", hash = "sha256:ad77c13037d3402fbeffda07d51e3f228ba078d1c7096a73759c9419ea031bf4"},
+    {file = "mypy-1.0.0-cp310-cp310-macosx_10_9_x86_64.whl", hash = "sha256:e0626db16705ab9f7fa6c249c017c887baf20738ce7f9129da162bb3075fc1af"},
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+    {file = "mypy-1.0.0-cp310-cp310-win_amd64.whl", hash = "sha256:7cc2c01dfc5a3cbddfa6c13f530ef3b95292f926329929001d45e124342cd6b7"},
+    {file = "mypy-1.0.0-cp311-cp311-macosx_10_9_x86_64.whl", hash = "sha256:14d776869a3e6c89c17eb943100f7868f677703c8a4e00b3803918f86aafbc52"},
+    {file = "mypy-1.0.0-cp311-cp311-macosx_11_0_arm64.whl", hash = "sha256:bb2782a036d9eb6b5a6efcdda0986774bf798beef86a62da86cb73e2a10b423d"},
+    {file = "mypy-1.0.0-cp311-cp311-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:5cfca124f0ac6707747544c127880893ad72a656e136adc935c8600740b21ff5"},
+    {file = "mypy-1.0.0-cp311-cp311-musllinux_1_1_x86_64.whl", hash = "sha256:8845125d0b7c57838a10fd8925b0f5f709d0e08568ce587cc862aacce453e3dd"},
+    {file = "mypy-1.0.0-cp311-cp311-win_amd64.whl", hash = "sha256:01b1b9e1ed40544ef486fa8ac022232ccc57109f379611633ede8e71630d07d2"},
+    {file = "mypy-1.0.0-cp37-cp37m-macosx_10_9_x86_64.whl", hash = "sha256:c7cf862aef988b5fbaa17764ad1d21b4831436701c7d2b653156a9497d92c83c"},
+    {file = "mypy-1.0.0-cp37-cp37m-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:5cd187d92b6939617f1168a4fe68f68add749902c010e66fe574c165c742ed88"},
+    {file = "mypy-1.0.0-cp37-cp37m-musllinux_1_1_x86_64.whl", hash = "sha256:4e5175026618c178dfba6188228b845b64131034ab3ba52acaffa8f6c361f805"},
+    {file = "mypy-1.0.0-cp37-cp37m-win_amd64.whl", hash = "sha256:2f6ac8c87e046dc18c7d1d7f6653a66787a4555085b056fe2d599f1f1a2a2d21"},
+    {file = "mypy-1.0.0-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:7306edca1c6f1b5fa0bc9aa645e6ac8393014fa82d0fa180d0ebc990ebe15964"},
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+    {file = "mypy-1.0.0-cp39-cp39-macosx_11_0_arm64.whl", hash = "sha256:5e398652d005a198a7f3c132426b33c6b85d98aa7dc852137a2a3be8890c4072"},
+    {file = "mypy-1.0.0-cp39-cp39-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:be78077064d016bc1b639c2cbcc5be945b47b4261a4f4b7d8923f6c69c5c9457"},
+    {file = "mypy-1.0.0-cp39-cp39-musllinux_1_1_x86_64.whl", hash = "sha256:92024447a339400ea00ac228369cd242e988dd775640755fa4ac0c126e49bb74"},
+    {file = "mypy-1.0.0-cp39-cp39-win_amd64.whl", hash = "sha256:fe523fcbd52c05040c7bee370d66fee8373c5972171e4fbc323153433198592d"},
+    {file = "mypy-1.0.0-py3-none-any.whl", hash = "sha256:2efa963bdddb27cb4a0d42545cd137a8d2b883bd181bbc4525b568ef6eca258f"},
+    {file = "mypy-1.0.0.tar.gz", hash = "sha256:f34495079c8d9da05b183f9f7daec2878280c2ad7cc81da686ef0b484cea2ecf"},
 ]
 
 [package.dependencies]
@@ -1186,6 +1188,7 @@ typing-extensions = ">=3.10"
 
 [package.extras]
 dmypy = ["psutil (>=4.0)"]
+install-types = ["pip"]
 python2 = ["typed-ast (>=1.4.0,<2)"]
 reports = ["lxml"]
 
@@ -1203,18 +1206,18 @@ files = [
 
 [[package]]
 name = "mypy-zope"
-version = "0.3.11"
+version = "0.9.0"
 description = "Plugin for mypy to support zope interfaces"
 category = "dev"
 optional = false
 python-versions = "*"
 files = [
-    {file = "mypy-zope-0.3.11.tar.gz", hash = "sha256:d4255f9f04d48c79083bbd4e2fea06513a6ac7b8de06f8c4ce563fd85142ca05"},
-    {file = "mypy_zope-0.3.11-py3-none-any.whl", hash = "sha256:ec080a6508d1f7805c8d2054f9fdd13c849742ce96803519e1fdfa3d3cab7140"},
+    {file = "mypy-zope-0.9.0.tar.gz", hash = "sha256:88bf6cd056e38b338e6956055958a7805b4ff84404ccd99e29883a3647a1aeb3"},
+    {file = "mypy_zope-0.9.0-py3-none-any.whl", hash = "sha256:e1bb4b57084f76ff8a154a3e07880a1af2ac6536c491dad4b143d529f72c5d15"},
 ]
 
 [package.dependencies]
-mypy = "0.981"
+mypy = "1.0.0"
 "zope.interface" = "*"
 "zope.schema" = "*"
 
@@ -1705,7 +1708,7 @@ files = [
 cffi = ">=1.4.1"
 
 [package.extras]
-docs = ["sphinx (>=1.6.5)", "sphinx-rtd-theme"]
+docs = ["sphinx (>=1.6.5)", "sphinx_rtd_theme"]
 tests = ["hypothesis (>=3.27.0)", "pytest (>=3.2.1,!=3.3.0)"]
 
 [[package]]
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 57a6854b1e..cf12b55d21 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -201,7 +201,7 @@ class AuthHandler:
         for auth_checker_class in INTERACTIVE_AUTH_CHECKERS:
             inst = auth_checker_class(hs)
             if inst.is_enabled():
-                self.checkers[inst.AUTH_TYPE] = inst  # type: ignore
+                self.checkers[inst.AUTH_TYPE] = inst
 
         self.bcrypt_rounds = hs.config.registration.bcrypt_rounds
 
diff --git a/synapse/handlers/ui_auth/checkers.py b/synapse/handlers/ui_auth/checkers.py
index 332edcca24..78a75bfed6 100644
--- a/synapse/handlers/ui_auth/checkers.py
+++ b/synapse/handlers/ui_auth/checkers.py
@@ -13,7 +13,8 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Any
+from abc import ABC, abstractmethod
+from typing import TYPE_CHECKING, Any, ClassVar, Sequence, Type
 
 from twisted.web.client import PartialDownloadError
 
@@ -27,19 +28,28 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-class UserInteractiveAuthChecker:
+class UserInteractiveAuthChecker(ABC):
     """Abstract base class for an interactive auth checker"""
 
-    def __init__(self, hs: "HomeServer"):
+    # This should really be an "abstract class property", i.e. it should
+    # be an error to instantiate a subclass that doesn't specify an AUTH_TYPE.
+    # But calling this a `ClassVar` is simpler than a decorator stack of
+    # @property @abstractmethod and @classmethod (if that's even the right order).
+    AUTH_TYPE: ClassVar[str]
+
+    def __init__(self, hs: "HomeServer"):  # noqa: B027
         pass
 
+    @abstractmethod
     def is_enabled(self) -> bool:
         """Check if the configuration of the homeserver allows this checker to work
 
         Returns:
             True if this login type is enabled.
         """
+        raise NotImplementedError()
 
+    @abstractmethod
     async def check_auth(self, authdict: dict, clientip: str) -> Any:
         """Given the authentication dict from the client, attempt to check this step
 
@@ -304,7 +314,7 @@ class RegistrationTokenAuthChecker(UserInteractiveAuthChecker):
             )
 
 
-INTERACTIVE_AUTH_CHECKERS = [
+INTERACTIVE_AUTH_CHECKERS: Sequence[Type[UserInteractiveAuthChecker]] = [
     DummyAuthChecker,
     TermsAuthChecker,
     RecaptchaAuthChecker,
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index b92f1d3d1a..312aab4dcc 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -1267,7 +1267,7 @@ class MatrixFederationHttpClient:
 def _flatten_response_never_received(e: BaseException) -> str:
     if hasattr(e, "reasons"):
         reasons = ", ".join(
-            _flatten_response_never_received(f.value) for f in e.reasons  # type: ignore[attr-defined]
+            _flatten_response_never_received(f.value) for f in e.reasons
         )
 
         return "%s:[%s]" % (type(e).__name__, reasons)
diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py
index 6c7cf1b294..5aed71262f 100644
--- a/synapse/logging/opentracing.py
+++ b/synapse/logging/opentracing.py
@@ -188,7 +188,7 @@ from typing import (
 )
 
 import attr
-from typing_extensions import ParamSpec
+from typing_extensions import Concatenate, ParamSpec
 
 from twisted.internet import defer
 from twisted.web.http import Request
@@ -445,7 +445,7 @@ def init_tracer(hs: "HomeServer") -> None:
         opentracing = None  # type: ignore[assignment]
         return
 
-    if not opentracing or not JaegerConfig:
+    if opentracing is None or JaegerConfig is None:
         raise ConfigError(
             "The server has been configured to use opentracing but opentracing is not "
             "installed."
@@ -872,7 +872,7 @@ def extract_text_map(carrier: Dict[str, str]) -> Optional["opentracing.SpanConte
 
 def _custom_sync_async_decorator(
     func: Callable[P, R],
-    wrapping_logic: Callable[[Callable[P, R], Any, Any], ContextManager[None]],
+    wrapping_logic: Callable[Concatenate[Callable[P, R], P], ContextManager[None]],
 ) -> Callable[P, R]:
     """
     Decorates a function that is sync or async (coroutines), or that returns a Twisted
@@ -902,10 +902,14 @@ def _custom_sync_async_decorator(
     """
 
     if inspect.iscoroutinefunction(func):
-
+        # In this branch, R = Awaitable[RInner], for some other type RInner
         @wraps(func)
-        async def _wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
+        async def _wrapper(
+            *args: P.args, **kwargs: P.kwargs
+        ) -> Any:  # Return type is RInner
             with wrapping_logic(func, *args, **kwargs):
+                # type-ignore: func() returns R, but mypy doesn't know that R is
+                # Awaitable here.
                 return await func(*args, **kwargs)  # type: ignore[misc]
 
     else:
@@ -972,7 +976,11 @@ def trace_with_opname(
         if not opentracing:
             return func
 
-        return _custom_sync_async_decorator(func, _wrapping_logic)
+        # type-ignore: mypy seems to be confused by the ParamSpecs here.
+        # I think the problem is https://github.com/python/mypy/issues/12909
+        return _custom_sync_async_decorator(
+            func, _wrapping_logic  # type: ignore[arg-type]
+        )
 
     return _decorator
 
@@ -1018,7 +1026,9 @@ def tag_args(func: Callable[P, R]) -> Callable[P, R]:
         set_tag(SynapseTags.FUNC_KWARGS, str(kwargs))
         yield
 
-    return _custom_sync_async_decorator(func, _wrapping_logic)
+    # type-ignore: mypy seems to be confused by the ParamSpecs here.
+    # I think the problem is https://github.com/python/mypy/issues/12909
+    return _custom_sync_async_decorator(func, _wrapping_logic)  # type: ignore[arg-type]
 
 
 @contextlib.contextmanager
diff --git a/synapse/rest/media/v1/_base.py b/synapse/rest/media/v1/_base.py
index d30878f704..6e035afcce 100644
--- a/synapse/rest/media/v1/_base.py
+++ b/synapse/rest/media/v1/_base.py
@@ -16,6 +16,7 @@
 import logging
 import os
 import urllib
+from abc import ABC, abstractmethod
 from types import TracebackType
 from typing import Awaitable, Dict, Generator, List, Optional, Tuple, Type
 
@@ -284,13 +285,14 @@ async def respond_with_responder(
     finish_request(request)
 
 
-class Responder:
+class Responder(ABC):
     """Represents a response that can be streamed to the requester.
 
     Responder is a context manager which *must* be used, so that any resources
     held can be cleaned up.
     """
 
+    @abstractmethod
     def write_to_consumer(self, consumer: IConsumer) -> Awaitable:
         """Stream response into consumer
 
@@ -300,11 +302,12 @@ class Responder:
         Returns:
             Resolves once the response has finished being written
         """
+        raise NotImplementedError()
 
-    def __enter__(self) -> None:
+    def __enter__(self) -> None:  # noqa: B027
         pass
 
-    def __exit__(
+    def __exit__(  # noqa: B027
         self,
         exc_type: Optional[Type[BaseException]],
         exc_val: Optional[BaseException],
diff --git a/synapse/storage/engines/__init__.py b/synapse/storage/engines/__init__.py
index a182e8a098..d1ccb7390a 100644
--- a/synapse/storage/engines/__init__.py
+++ b/synapse/storage/engines/__init__.py
@@ -25,7 +25,7 @@ try:
 except ImportError:
 
     class PostgresEngine(BaseDatabaseEngine):  # type: ignore[no-redef]
-        def __new__(cls, *args: object, **kwargs: object) -> NoReturn:  # type: ignore[misc]
+        def __new__(cls, *args: object, **kwargs: object) -> NoReturn:
             raise RuntimeError(
                 f"Cannot create {cls.__name__} -- psycopg2 module is not installed"
             )
@@ -36,7 +36,7 @@ try:
 except ImportError:
 
     class Sqlite3Engine(BaseDatabaseEngine):  # type: ignore[no-redef]
-        def __new__(cls, *args: object, **kwargs: object) -> NoReturn:  # type: ignore[misc]
+        def __new__(cls, *args: object, **kwargs: object) -> NoReturn:
             raise RuntimeError(
                 f"Cannot create {cls.__name__} -- sqlite3 module is not installed"
             )
diff --git a/synapse/storage/types.py b/synapse/storage/types.py
index 0031df1e06..56a0048539 100644
--- a/synapse/storage/types.py
+++ b/synapse/storage/types.py
@@ -12,7 +12,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 from types import TracebackType
-from typing import Any, Iterator, List, Mapping, Optional, Sequence, Tuple, Type, Union
+from typing import (
+    Any,
+    Callable,
+    Iterator,
+    List,
+    Mapping,
+    Optional,
+    Sequence,
+    Tuple,
+    Type,
+    Union,
+)
 
 from typing_extensions import Protocol
 
@@ -112,15 +123,35 @@ class DBAPI2Module(Protocol):
     #   extends from this hierarchy. See
     #     https://docs.python.org/3/library/sqlite3.html?highlight=sqlite3#exceptions
     #     https://www.postgresql.org/docs/current/errcodes-appendix.html#ERRCODES-TABLE
-    Warning: Type[Exception]
-    Error: Type[Exception]
+    #
+    # Note: rather than
+    #     x: T
+    # we write
+    #     @property
+    #     def x(self) -> T: ...
+    # which expresses that the protocol attribute `x` is read-only. The mypy docs
+    #     https://mypy.readthedocs.io/en/latest/common_issues.html#covariant-subtyping-of-mutable-protocol-members-is-rejected
+    # explain why this is necessary for safety. TL;DR: we shouldn't be able to write
+    # to `x`, only read from it. See also https://github.com/python/mypy/issues/6002 .
+    @property
+    def Warning(self) -> Type[Exception]:
+        ...
+
+    @property
+    def Error(self) -> Type[Exception]:
+        ...
 
     # Errors are divided into `InterfaceError`s (something went wrong in the database
     # driver) and `DatabaseError`s (something went wrong in the database). These are
     # both subclasses of `Error`, but we can't currently express this in type
     # annotations due to https://github.com/python/mypy/issues/8397
-    InterfaceError: Type[Exception]
-    DatabaseError: Type[Exception]
+    @property
+    def InterfaceError(self) -> Type[Exception]:
+        ...
+
+    @property
+    def DatabaseError(self) -> Type[Exception]:
+        ...
 
     # Everything below is a subclass of `DatabaseError`.
 
@@ -128,7 +159,9 @@ class DBAPI2Module(Protocol):
     # - An integer was too big for its data type.
     # - An invalid date time was provided.
     # - A string contained a null code point.
-    DataError: Type[Exception]
+    @property
+    def DataError(self) -> Type[Exception]:
+        ...
 
     # Roughly: something went wrong in the database, but it's not within the application
     # programmer's control. Examples:
@@ -138,28 +171,45 @@ class DBAPI2Module(Protocol):
     # - A serialisation failure occurred.
     # - The database ran out of resources, such as storage, memory, connections, etc.
     # - The database encountered an error from the operating system.
-    OperationalError: Type[Exception]
+    @property
+    def OperationalError(self) -> Type[Exception]:
+        ...
 
     # Roughly: we've given the database data which breaks a rule we asked it to enforce.
     # Examples:
     # - Stop, criminal scum! You violated the foreign key constraint
     # - Also check constraints, non-null constraints, etc.
-    IntegrityError: Type[Exception]
+    @property
+    def IntegrityError(self) -> Type[Exception]:
+        ...
 
     # Roughly: something went wrong within the database server itself.
-    InternalError: Type[Exception]
+    @property
+    def InternalError(self) -> Type[Exception]:
+        ...
 
     # Roughly: the application did something silly that needs to be fixed. Examples:
     # - We don't have permissions to do something.
     # - We tried to create a table with duplicate column names.
     # - We tried to use a reserved name.
     # - We referred to a column that doesn't exist.
-    ProgrammingError: Type[Exception]
+    @property
+    def ProgrammingError(self) -> Type[Exception]:
+        ...
 
     # Roughly: we've tried to do something that this database doesn't support.
-    NotSupportedError: Type[Exception]
+    @property
+    def NotSupportedError(self) -> Type[Exception]:
+        ...
 
-    def connect(self, **parameters: object) -> Connection:
+    # We originally wrote
+    # def connect(self, *args, **kwargs) -> Connection: ...
+    # But mypy doesn't seem to like that because sqlite3.connect takes a mandatory
+    # positional argument. We can't make that part of the signature though, because
+    # psycopg2.connect doesn't have a mandatory positional argument. Instead, we use
+    # the following slightly unusual workaround.
+    @property
+    def connect(self) -> Callable[..., Connection]:
         ...
 
 
diff --git a/synapse/streams/__init__.py b/synapse/streams/__init__.py
index c6c8a0315c..8a48ffc48d 100644
--- a/synapse/streams/__init__.py
+++ b/synapse/streams/__init__.py
@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+from abc import ABC, abstractmethod
 from typing import Generic, List, Optional, Tuple, TypeVar
 
 from synapse.types import StrCollection, UserID
@@ -22,7 +22,8 @@ K = TypeVar("K")
 R = TypeVar("R")
 
 
-class EventSource(Generic[K, R]):
+class EventSource(ABC, Generic[K, R]):
+    @abstractmethod
     async def get_new_events(
         self,
         user: UserID,
@@ -32,4 +33,4 @@ class EventSource(Generic[K, R]):
         is_guest: bool,
         explicit_room_id: Optional[str] = None,
     ) -> Tuple[List[R], K]:
-        ...
+        raise NotImplementedError()
diff --git a/tests/handlers/test_register.py b/tests/handlers/test_register.py
index 782ef09cf4..1db99b3c00 100644
--- a/tests/handlers/test_register.py
+++ b/tests/handlers/test_register.py
@@ -62,7 +62,7 @@ class TestSpamChecker:
         request_info: Collection[Tuple[str, str]],
         auth_provider_id: Optional[str],
     ) -> RegistrationBehaviour:
-        pass
+        return RegistrationBehaviour.ALLOW
 
 
 class DenyAll(TestSpamChecker):
@@ -111,7 +111,7 @@ class TestLegacyRegistrationSpamChecker:
         username: Optional[str],
         request_info: Collection[Tuple[str, str]],
     ) -> RegistrationBehaviour:
-        pass
+        return RegistrationBehaviour.ALLOW
 
 
 class LegacyAllowAll(TestLegacyRegistrationSpamChecker):
diff --git a/tests/http/federation/test_matrix_federation_agent.py b/tests/http/federation/test_matrix_federation_agent.py
index acfdcd3bca..d27422515c 100644
--- a/tests/http/federation/test_matrix_federation_agent.py
+++ b/tests/http/federation/test_matrix_federation_agent.py
@@ -63,7 +63,7 @@ from tests.http import (
     get_test_ca_cert_file,
 )
 from tests.server import FakeTransport, ThreadedMemoryReactorClock
-from tests.utils import default_config
+from tests.utils import checked_cast, default_config
 
 logger = logging.getLogger(__name__)
 
@@ -146,8 +146,10 @@ class MatrixFederationAgentTests(unittest.TestCase):
         #
         # Normally this would be done by the TCP socket code in Twisted, but we are
         # stubbing that out here.
-        client_protocol = client_factory.buildProtocol(dummy_address)
-        assert isinstance(client_protocol, _WrappingProtocol)
+        # NB: we use a checked_cast here to workaround https://github.com/Shoobx/mypy-zope/issues/91)
+        client_protocol = checked_cast(
+            _WrappingProtocol, client_factory.buildProtocol(dummy_address)
+        )
         client_protocol.makeConnection(
             FakeTransport(server_protocol, self.reactor, client_protocol)
         )
@@ -446,7 +448,6 @@ class MatrixFederationAgentTests(unittest.TestCase):
         server_ssl_protocol = _wrap_server_factory_for_tls(
             _get_test_protocol_factory()
         ).buildProtocol(dummy_address)
-        assert isinstance(server_ssl_protocol, TLSMemoryBIOProtocol)
 
         # Tell the HTTP server to send outgoing traffic back via the proxy's transport.
         proxy_server_transport = proxy_server.transport
@@ -1529,7 +1530,7 @@ def _check_logcontext(context: LoggingContextOrSentinel) -> None:
 
 def _wrap_server_factory_for_tls(
     factory: IProtocolFactory, sanlist: Optional[List[bytes]] = None
-) -> IProtocolFactory:
+) -> TLSMemoryBIOFactory:
     """Wrap an existing Protocol Factory with a test TLSMemoryBIOFactory
     The resultant factory will create a TLS server which presents a certificate
     signed by our test CA, valid for the domains in `sanlist`
diff --git a/tests/http/test_proxyagent.py b/tests/http/test_proxyagent.py
index a817940730..22fdc7f5f2 100644
--- a/tests/http/test_proxyagent.py
+++ b/tests/http/test_proxyagent.py
@@ -43,6 +43,7 @@ from tests.http import (
 )
 from tests.server import FakeTransport, ThreadedMemoryReactorClock
 from tests.unittest import TestCase
+from tests.utils import checked_cast
 
 logger = logging.getLogger(__name__)
 
@@ -620,7 +621,6 @@ class MatrixFederationAgentTests(TestCase):
         server_ssl_protocol = _wrap_server_factory_for_tls(
             _get_test_protocol_factory()
         ).buildProtocol(dummy_address)
-        assert isinstance(server_ssl_protocol, TLSMemoryBIOProtocol)
 
         # Tell the HTTP server to send outgoing traffic back via the proxy's transport.
         proxy_server_transport = proxy_server.transport
@@ -757,12 +757,14 @@ class MatrixFederationAgentTests(TestCase):
         assert isinstance(proxy_server, HTTPChannel)
 
         # fish the transports back out so that we can do the old switcheroo
-        s2c_transport = proxy_server.transport
-        assert isinstance(s2c_transport, FakeTransport)
-        client_protocol = s2c_transport.other
-        assert isinstance(client_protocol, _WrappingProtocol)
-        c2s_transport = client_protocol.transport
-        assert isinstance(c2s_transport, FakeTransport)
+        # To help mypy out with the various Protocols and wrappers and mocks, we do
+        # some explicit casting. Without the casts, we hit the bug I reported at
+        # https://github.com/Shoobx/mypy-zope/issues/91 .
+        # We also double-checked these casts at runtime (test-time) because I found it
+        # quite confusing to deduce these types in the first place!
+        s2c_transport = checked_cast(FakeTransport, proxy_server.transport)
+        client_protocol = checked_cast(_WrappingProtocol, s2c_transport.other)
+        c2s_transport = checked_cast(FakeTransport, client_protocol.transport)
 
         # the FakeTransport is async, so we need to pump the reactor
         self.reactor.advance(0)
@@ -822,9 +824,9 @@ class MatrixFederationAgentTests(TestCase):
     @patch.dict(os.environ, {"http_proxy": "proxy.com:8888"})
     def test_proxy_with_no_scheme(self) -> None:
         http_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(http_proxy_agent.http_proxy_endpoint, HostnameEndpoint)
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._hostStr, "proxy.com")
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._port, 8888)
+        proxy_ep = checked_cast(HostnameEndpoint, http_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._port, 8888)
 
     @patch.dict(os.environ, {"http_proxy": "socks://proxy.com:8888"})
     def test_proxy_with_unsupported_scheme(self) -> None:
@@ -834,25 +836,21 @@ class MatrixFederationAgentTests(TestCase):
     @patch.dict(os.environ, {"http_proxy": "http://proxy.com:8888"})
     def test_proxy_with_http_scheme(self) -> None:
         http_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(http_proxy_agent.http_proxy_endpoint, HostnameEndpoint)
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._hostStr, "proxy.com")
-        self.assertEqual(http_proxy_agent.http_proxy_endpoint._port, 8888)
+        proxy_ep = checked_cast(HostnameEndpoint, http_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._port, 8888)
 
     @patch.dict(os.environ, {"http_proxy": "https://proxy.com:8888"})
     def test_proxy_with_https_scheme(self) -> None:
         https_proxy_agent = ProxyAgent(self.reactor, use_proxy=True)
-        assert isinstance(https_proxy_agent.http_proxy_endpoint, _WrapperEndpoint)
-        self.assertEqual(
-            https_proxy_agent.http_proxy_endpoint._wrappedEndpoint._hostStr, "proxy.com"
-        )
-        self.assertEqual(
-            https_proxy_agent.http_proxy_endpoint._wrappedEndpoint._port, 8888
-        )
+        proxy_ep = checked_cast(_WrapperEndpoint, https_proxy_agent.http_proxy_endpoint)
+        self.assertEqual(proxy_ep._wrappedEndpoint._hostStr, "proxy.com")
+        self.assertEqual(proxy_ep._wrappedEndpoint._port, 8888)
 
 
 def _wrap_server_factory_for_tls(
     factory: IProtocolFactory, sanlist: Optional[List[bytes]] = None
-) -> IProtocolFactory:
+) -> TLSMemoryBIOFactory:
     """Wrap an existing Protocol Factory with a test TLSMemoryBIOFactory
 
     The resultant factory will create a TLS server which presents a certificate
diff --git a/tests/logging/test_remote_handler.py b/tests/logging/test_remote_handler.py
index c08954d887..5191e31a8a 100644
--- a/tests/logging/test_remote_handler.py
+++ b/tests/logging/test_remote_handler.py
@@ -21,6 +21,7 @@ from synapse.logging import RemoteHandler
 from tests.logging import LoggerCleanupMixin
 from tests.server import FakeTransport, get_clock
 from tests.unittest import TestCase
+from tests.utils import checked_cast
 
 
 def connect_logging_client(
@@ -56,8 +57,8 @@ class RemoteHandlerTestCase(LoggerCleanupMixin, TestCase):
         client, server = connect_logging_client(self.reactor, 0)
 
         # Trigger data being sent
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # One log message, with a single trailing newline
         logs = server.data.decode("utf8").splitlines()
@@ -89,8 +90,8 @@ class RemoteHandlerTestCase(LoggerCleanupMixin, TestCase):
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # Only the 7 infos made it through, the debugs were elided
         logs = server.data.splitlines()
@@ -123,8 +124,8 @@ class RemoteHandlerTestCase(LoggerCleanupMixin, TestCase):
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # The 10 warnings made it through, the debugs and infos were elided
         logs = server.data.splitlines()
@@ -148,8 +149,8 @@ class RemoteHandlerTestCase(LoggerCleanupMixin, TestCase):
 
         # Allow the reconnection
         client, server = connect_logging_client(self.reactor, 0)
-        assert isinstance(client.transport, FakeTransport)
-        client.transport.flush()
+        client_transport = checked_cast(FakeTransport, client.transport)
+        client_transport.flush()
 
         # The first five and last five warnings made it through, the debugs and
         # infos were elided
diff --git a/tests/rest/client/test_auth.py b/tests/rest/client/test_auth.py
index 208ec44829..f4e1e7de43 100644
--- a/tests/rest/client/test_auth.py
+++ b/tests/rest/client/test_auth.py
@@ -43,6 +43,9 @@ class DummyRecaptchaChecker(UserInteractiveAuthChecker):
         super().__init__(hs)
         self.recaptcha_attempts: List[Tuple[dict, str]] = []
 
+    def is_enabled(self) -> bool:
+        return True
+
     def check_auth(self, authdict: dict, clientip: str) -> Any:
         self.recaptcha_attempts.append((authdict, clientip))
         return succeed(True)
diff --git a/tests/rest/client/test_third_party_rules.py b/tests/rest/client/test_third_party_rules.py
index 3325d43a2f..5fa3440691 100644
--- a/tests/rest/client/test_third_party_rules.py
+++ b/tests/rest/client/test_third_party_rules.py
@@ -425,7 +425,7 @@ class ThirdPartyRulesTestCase(unittest.FederatingHomeserverTestCase):
         async def test_fn(
             event: EventBase, state_events: StateMap[EventBase]
         ) -> Tuple[bool, Optional[JsonDict]]:
-            if event.is_state and event.type == EventTypes.PowerLevels:
+            if event.is_state() and event.type == EventTypes.PowerLevels:
                 await api.create_and_send_event_into_room(
                     {
                         "room_id": event.room_id,
diff --git a/tests/utils.py b/tests/utils.py
index 15fabbc2d0..a0ac11bc5c 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -15,7 +15,7 @@
 
 import atexit
 import os
-from typing import Any, Callable, Dict, List, Tuple, Union, overload
+from typing import Any, Callable, Dict, List, Tuple, Type, TypeVar, Union, overload
 
 import attr
 from typing_extensions import Literal, ParamSpec
@@ -341,3 +341,27 @@ async def create_room(hs: HomeServer, room_id: str, creator_id: str) -> None:
     context = await unpersisted_context.persist(event)
 
     await persistence_store.persist_event(event, context)
+
+
+T = TypeVar("T")
+
+
+def checked_cast(type: Type[T], x: object) -> T:
+    """A version of typing.cast that is checked at runtime.
+
+    We have our own function for this for two reasons:
+
+    1. typing.cast itself is deliberately a no-op at runtime, see
+       https://docs.python.org/3/library/typing.html#typing.cast
+    2. To help workaround a mypy-zope bug https://github.com/Shoobx/mypy-zope/issues/91
+       where mypy would erroneously consider `isinstance(x, type)` to be false in all
+       circumstances.
+
+    For this to make sense, `T` needs to be something that `isinstance` can check; see
+        https://docs.python.org/3/library/functions.html?highlight=isinstance#isinstance
+        https://docs.python.org/3/glossary.html#term-abstract-base-class
+        https://docs.python.org/3/library/typing.html#typing.runtime_checkable
+    for more details.
+    """
+    assert isinstance(x, type)
+    return x