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/synapse/metrics/jemalloc.py b/synapse/metrics/jemalloc.py

index bd25985686..321ff58083 100644
--- a/synapse/metrics/jemalloc.py
+++ b/synapse/metrics/jemalloc.py
@@ -23,11 +23,10 @@ import ctypes
 import logging
 import os
 import re
-from typing import Iterable, Optional, overload
+from typing import Iterable, Literal, Optional, overload
 
 import attr
 from prometheus_client import REGISTRY, Metric
-from typing_extensions import Literal
 
 from synapse.metrics import GaugeMetricFamily
 from synapse.metrics._types import Collector