Add support for Python 3.12.1

[edmorley]

Release announcement:
https://blog.python.org/2023/12/python-3121-is-now-available.html
https://www.python.org/downloads/release/python-3121/
https://docs.python.org/release/3.12.1/whatsnew/changelog.html#python-3-12-1-final

Binary builds:
https://github.com/heroku/heroku-buildpack-python/actions/runs/7138923192

GUS-W-14233002.

[edmorley]

Updated versions on:
https://devcenter.heroku.com/articles/python-support#specifying-a-python-version
https://devcenter.heroku.com/articles/python-runtimes#selecting-a-runtime

Published changelog entry:
https://devcenter.heroku.com/changelog-items/2766