Draft: mv to pyproject.toml

Slowly but steadily, the python ecosystem is moving to pyproject.toml. For us, the main benefit currently is having one less file (it replaces both setup.py and setup.cfg).

For a while, pyproject.toml had the downside of not supporting editable installs. This has been fixed with PEP 660 which is suppored by pip >= 21.3 and setuptools >= 64.

TODO:

• Ensure that supported versions of pip and setuptools are available
• Port setuptools config
• Port isort config
• Port pytest config
• Port coverage config
• Flake8 itself does not yet support pyproject.toml, but a plugin is available