build(meta): add Python 3.12 package classifier

[shiftinv]

Summary

Adds the Python :: 3.12 package classifier, and reverts the temporary CI changes (#1117) now that there's a stable aiohttp release compatible with 3.12.

Blocked by #1128.

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, ...)

[shiftinv]

This is no longer blocked now. Still, I'm going to give it a little bit more time and see how things work out in the long run - in the end this is mainly a cosmetic change, anyway.