ci: start testing with python 3.12

[shiftinv]

Summary

Adds 3.12 to the CI version matrix, and updates libcst, pytest, and typing_extensions to the latest versions as they weren't compatible before.
As a temporary workaround, this installs the current aiohttp 3.9.0b0 prerelease, but only in CI; all currently released stable versions don't build on 3.12 yet.

No code changes specifically for 3.12 in this PR, I'm keeping those separate for another PR.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)