Support Python 3.12

[hf-kklein]

The package shall support Python 3.12.
I think we don't need any code adjustments, just metadata and stuff:

• unittest Python 3.11 AND Python 3.12
• switch other CI actions to Python 3.12 only (no need to lint Python 3.11 AND Python 3.12)
• add 3.12 to pyproject.toml