Improve build / install

[Dieter Werthmüller]

• Move from setup.py to pyproject.toml/setup.cfg (don't see the point atm for pure python package)
• Use micromamba for GHA (miniforge is just OK)
• Create a proper environment.yml (not atm; users should use pip or conda; the requirements-files are just to develop.)
• python -m pip install --no-build-isolation --no-deps -e .
• (Related): Add cffinit (don't see the point atm)
• (Related): Add CoC (if the community grows significantly, re-open this)