Add support for Python 3.12

[hugovk]

Summary

Follow on from encode/httpcore#807.

The third and final Python 3.12 release candidate is out! 🚀

Call to action

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.12 compatibilities during this phase, and where necessary publish Python 3.12 wheels on PyPI to be ready for the final release of 3.12.0.

Python 3.12.0 final is due out in two weeks: 2023-10-02.

See also https://dev.to/hugovk/help-test-python-312-beta-1508/

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.