Switch as much as possible to pyproject.toml

[MichaelClerx]

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/

• [ ] setup.py
• [ ] coverage https://coverage.readthedocs.io/en/latest/config.html Python 3.11 and later
• [ ] Flake8 not playing ball? https://github.com/PyCQA/flake8/issues/234
• [ ] manifest.in --> don't think so
• [ ] readthedocs --> don't think so