Model Python version as a PubGrub package

[charliermarsh]

Summary

This PR modifies the resolver to treat the Python version as a package, which allows for better error messages (since we no longer treat incompatible packages as if they "don't exist at all").

There are a few tricky pieces here...

First, we need to track both the interpreter's Python version and the target Python version, because we support resolving for other versions via --python 3.7.

Second, we allow using incompatible wheels during resolution, as long as there's a compatible source distribution. So we still need to test for requires-python compatibility when selecting distributions.

This could use more testing, but it feels like an area where packse would be more productive than writing PyPI tests.

Closes #406.

[charliermarsh]

This is actually harder than I thought. Consider the case of resolving NumPy with --python-version 3.8. In that case, we want to skip building all the source distributions for versions later than 1.24.4. But we want to return them to PubGrub so that they're modeled as incompatibilities and not ignored entirely. So we need some new abstraction around Candidate to indicate whether it's compatible or incompatible.

[charliermarsh]

I think I know how to solve but will be a bit more work, so marking as draft.

[charliermarsh]

Okay, should be ready for review.