CI setup

[ffelten]

• Rely on pre-commit to launch all the CI utils: flake8, black, ... I took the configs from Gymnasium mainly.
• Moved things from setup.py to pyproject.toml as in Gymnasium;
• Fixed all the small issues.

Testing

pip install pre-commit
pre-commit install
pre-commit run --all-files

Normally everything should be green.

I left some TODOs for the renaming of the package which I didn't do in this PR.