CI stops at Python 3.10

[jaraco]

Currently, the CI runs on Python 3.10 only. Please consider keeping CI up-to-date with stable releases if not pre-releases. I maintain this expectation in jaraco/skeleton, and you're welcome to merge/cherrypick changes from there if that helps.

[woodruffw]

I've folded this into #1029, since 3.12 has also been released in the meantime and has required some more involved changes (due to the removal of setuptools from default venvs).

[sigmavirus24]

Closed with #1029