Modernize repo

[leplatrem]

• [x] Replaced setup.cfg and setup.py by pyproject.toml
• [x] Replaced flake8 by ruff
• [x] Use requirements.in instead of list in setup.py
• [x] Replaced manually created requirements.txt by pip-compile
• [x] Remove dev-requirements.txt file and use extra dependencies instead
• [x] Use version from git tag (setuptools-scm)
• [x] Publish to Pypi on git tag
• [x] Add config to categorize changes in autogenerated changelog
• [x] Update contributing docs
• [x] Update release instructions
• [x] Rename master to main
• [x] Replace tox with GH actions
• [x] Move package to src
• [x] Wait for Pypa publish action to be allowed https://bugzilla.mozilla.org/show_bug.cgi?id=1877225

Things lost with this PR:

• No more built docs
• No more changelog (now on GH releases)
• Description on Pypi does not contain changelog contents anymore