The PR #4707 introduced the notion of "version narrowing," where a
Requires-Python constraint was _possibly_ narrowed whenever the
universal resolver created a fork. The version narrowing would occur
when the fork was a result of a marker expression on `python_version`
that is *stricter* than the configured `Requires-Python` (via, say,
`pyproject.toml`).
The crucial conceptual change made by #4707 is therefore that
`Requires-Python` is no longer an invariant configuration of resolution,
but rather a mutable constraint that can vary from fork to fork. This in
turn can result in some cases, such as in #4885, where different
versions of dependencies are selected. We aren't sure whether we can fix
those or not, with version narrowing, so for now, we do this revert to
restore the previous behavior and we'll try to address the version
narrowing some other time.
This also adds the case from #4885 as a regression test, ensuring that
we don't break that in the future. I confirmed that with version
narrowing, this test outputs duplicate distributions. Without narrowing,
there are no duplicates.
Ref #4707, Fixes#4885
