Use pep517 to extract metadata from sdists.

[tomprince]

This uses DavHau/mach-nix#401 to extract metadata from sdists, instead of a patched setuputils.

This also drops support for python 2 and python 3.6, as the code paths in mach-nix do not currently work with those versions.

An incomplete list of things that still need to be done:

• All the data for at least sdists containing a build-system section in pyproject.toml needs to be updated.
  • I suspect that this can't be done in github-actions.
• When doing some testing via pushing to github (in Use pep517 #4), I was often seeing locking errors in nix, which probably needs to be addressed before this lands.
  • Given that extracting requirements uses import-from-derivation as a step during building, and that metadata-env and build-env are likely going to be identical between a bunch of packages, I suspect those errors are likely related.
• There should probably be some spot-checking done of the generated data. I did this for one package: when the extraction worked, the data looked reasonable, but there were cases where I would expect it to work and didn't.
• There is some code to extract errors in updater/crawl_sdist_deps.py, that will need to be updated to deal with the new structure of extraction and its output.

Note: I currently have no plans to continue this work in the future.