Add support for Python 3.12 and drop EOL <= 3.7

[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 EOL Python <= 3.7, helps #313.

See also:

• https://dev.to/hugovk/help-test-python-312-beta-1508/
• https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html
• https://setuptools.pypa.io/en/latest/build_meta.html

[khneal]

This would be nice to have.
@jamesls - are you available to review+approve+merge+release?