Add support for Python 3.12

[hugovk]

The 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.

Also:

• Drop support for EOL Python 3.7: https://devguide.python.org/versions/
• Upgrade syntax with pyupgrade --py38-plus

[eliben]

Thanks for the contribution. This PR is trying to do a bit too much. Please split it:

1. One PR should just update the versions
2. A followup PR can then apply stylistic changes, but without touching the ply directory/module since this is vendored code I'm not modifying

[hugovk]

Updated!