Cleanup Python 3.8 leftovers (#17967)

Some small cleanups after Python3.8 became EOL.

- Move some type imports from `typing_extensions` to `typing`
- Remove the `abi3-py38` feature from pyo3

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

1 files changed, 11 insertions, 2 deletions

diff --git a/synapse/util/caches/dictionary_cache.py b/synapse/util/caches/dictionary_cache.py

index 1e6696332f..14bd3ba3b0 100644
--- a/synapse/util/caches/dictionary_cache.py
+++ b/synapse/util/caches/dictionary_cache.py
@@ -21,10 +21,19 @@
 import enum
 import logging
 import threading
-from typing import Dict, Generic, Iterable, Optional, Set, Tuple, TypeVar, Union
+from typing import (
+    Dict,
+    Generic,
+    Iterable,
+    Literal,
+    Optional,
+    Set,
+    Tuple,
+    TypeVar,
+    Union,
+)
 
 import attr
-from typing_extensions import Literal
 
 from synapse.util.caches.lrucache import LruCache
 from synapse.util.caches.treecache import TreeCache