Move to pyproject.toml and update testing routine

[martinvonk]

• Move setup.py, pytest.ini and requirements.txt to pyproject.toml
• Use tox for testing locally and on GitHub CI
• Fix some linting and formatting issues

[martinvonk]

I can imagine that we don't want to test on all python versions from 3.7 - 3.11. Maybe only do one? or the newest and oldest?

[martinvonk]

I can imagine that we don't want to test on all python versions from 3.7 - 3.11. Maybe only do one? or the newest and oldest?

As discussed with @OnnoEbbens , we now only test for python 3.10 and 3.9