Migrate to pyproject.toml

[nijel]

Proposed changes

Use pyproject.toml instead of setup.py.

Blockers:

• CI configuration is not adjusted (should be easy for most of the parts, it would just cause conflicts in the long term as this is not for immediate merge)
• Figure out minimal versions testing (Add a resolver option to use the specified minimum version for a dependency pypa/pip#8085 / Add an option for minimum version selection pypa/pip#11336 might be a solution here)
• Renovate support for this is missing, see New python manager for PEP621 renovatebot/renovate#10187

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information