Update supported Python versions

Currently:

- The Gitlab CI is tested with Python 3.7 to 3.10.
- Black is configured to work on Python 3.7 to 3.10
- `pyproject.toml` / `setup.py` / `setup.cfg`

I would suggest we drop support for Python 3.7 and lower (deprecated since [June 2023](https://devguide.python.org/versions/)) and adjust everything to Python 3.8 to 3.12.