WIP: use pep517 to extract metadata

[tomprince]

Fixes #319.

TODO:

• Properly parse requires-dist (since this includes the equivalent of the old install_requires and extra_requires). This should perhaps be done in the python, before writing out the metadata.
• Provide code paths for use by pypi-deps-db.
• Provide code paths for use by flake's extract-reqs.
• Remove obsolete code.
• Decide on whether to drop python2 support. If so, document that change; if not, provide fallback code to handle that case.
• ....

[tomprince]

Closing in favour of #401, which is from a repo that isn't likely to be deleted soon.