Remove various bits of compatibility code for Python <3.6 (#9879)

I went through and removed a bunch of cruft that was lying around for compatibility with old Python versions. This PR also will now prevent Synapse from starting unless you're running Python 3.6+.

2 files changed, 7 insertions, 10 deletions

diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py

index d472676acf..6b68d8720c 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -114,7 +114,7 @@ def db_to_json(db_content: Union[memoryview, bytes, bytearray, str]) -> Any:
         db_content = db_content.tobytes()
 
     # Decode it to a Unicode string before feeding it to the JSON decoder, since
-    # Python 3.5 does not support deserializing bytes.
+    # it only supports handling strings
     if isinstance(db_content, (bytes, bytearray)):
         db_content = db_content.decode("utf8")
 
diff --git a/synapse/storage/database.py b/synapse/storage/database.py

index 9452368bf0..bd39c095af 100644
--- a/synapse/storage/database.py
+++ b/synapse/storage/database.py
@@ -171,10 +171,7 @@ class LoggingDatabaseConnection:
 
 
 # The type of entry which goes on our after_callbacks and exception_callbacks lists.
-#
-# Python 3.5.2 doesn't support Callable with an ellipsis, so we wrap it in quotes so
-# that mypy sees the type but the runtime python doesn't.
-_CallbackListEntry = Tuple["Callable[..., None]", Iterable[Any], Dict[str, Any]]
+_CallbackListEntry = Tuple[Callable[..., None], Iterable[Any], Dict[str, Any]]
 
 
 R = TypeVar("R")
@@ -221,7 +218,7 @@ class LoggingTransaction:
         self.after_callbacks = after_callbacks
         self.exception_callbacks = exception_callbacks
 
-    def call_after(self, callback: "Callable[..., None]", *args: Any, **kwargs: Any):
+    def call_after(self, callback: Callable[..., None], *args: Any, **kwargs: Any):
         """Call the given callback on the main twisted thread after the
         transaction has finished. Used to invalidate the caches on the
         correct thread.
@@ -233,7 +230,7 @@ class LoggingTransaction:
         self.after_callbacks.append((callback, args, kwargs))
 
     def call_on_exception(
-        self, callback: "Callable[..., None]", *args: Any, **kwargs: Any
+        self, callback: Callable[..., None], *args: Any, **kwargs: Any
     ):
         # if self.exception_callbacks is None, that means that whatever constructed the
         # LoggingTransaction isn't expecting there to be any callbacks; assert that
@@ -485,7 +482,7 @@ class DatabasePool:
         desc: str,
         after_callbacks: List[_CallbackListEntry],
         exception_callbacks: List[_CallbackListEntry],
-        func: "Callable[..., R]",
+        func: Callable[..., R],
         *args: Any,
         **kwargs: Any,
     ) -> R:
@@ -618,7 +615,7 @@ class DatabasePool:
     async def runInteraction(
         self,
         desc: str,
-        func: "Callable[..., R]",
+        func: Callable[..., R],
         *args: Any,
         db_autocommit: bool = False,
         **kwargs: Any,
@@ -678,7 +675,7 @@ class DatabasePool:
 
     async def runWithConnection(
         self,
-        func: "Callable[..., R]",
+        func: Callable[..., R],
         *args: Any,
         db_autocommit: bool = False,
         **kwargs: Any,