WIP: extraction of requirements for pyproject.toml projects

[TyberiusPrime]

This is a WIP PR to finally get automated extraction of pyproject.toml projects.

Right now it's got two weaknesses:

• no tests (I have to do a deep dive into understanding how mach-nix is tested in the first place)
• and support only for the 'script_single' case of the requirements extractor - I'm not yet certain when the 'script' case (which seems to handle multiple python versions?) is being called.

Once I got pypy-debs-db running again, I'll see if I can stick the extractor in that, that should give us a better picture with regards on what it can and can't read.