use modern tooling

The following is a first plan to modernize the tooling for this project.

- [ ] use pyproject.toml instead of setup.cfg/setup.py
- [ ] use ruff to format and lint the source code (exclude some parts in the test where the formatting is important)
- [ ] increase test coverage to 100%
- [ ] also test patch versions of cpython (long running tests could be excluded for the patch versions)
- [ ] use pysource-codegen to search for new bugs

Feel free to add to this list if you have any other ideas.