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, 1 insertions, 2 deletions

diff --git a/tests/rest/client/test_media.py b/tests/rest/client/test_media.py

index 4060525efe..7d7dbd342b 100644
--- a/tests/rest/client/test_media.py
+++ b/tests/rest/client/test_media.py
@@ -24,14 +24,13 @@ import json
 import os
 import re
 import shutil
-from typing import Any, BinaryIO, Dict, List, Optional, Sequence, Tuple, Type
+from typing import Any, BinaryIO, ClassVar, Dict, List, Optional, Sequence, Tuple, Type
 from unittest.mock import MagicMock, Mock, patch
 from urllib import parse
 from urllib.parse import quote, urlencode
 
 from parameterized import parameterized, parameterized_class
 from PIL import Image as Image
-from typing_extensions import ClassVar
 
 from twisted.internet import defer
 from twisted.internet._resolver import HostResolution