DEP: Add Python 3.12

[snowman2]

• https://docs.python.org/3.12/whatsnew/3.12.html
• candidate 1: Monday, 2023-07-31: https://peps.python.org/pep-0693/ (when wheels can be built)
• Release 2023-10-02: https://devguide.python.org/versions/