Use intersection rather than union for `requires-python` (#5644)
## Summary
As-is, if you have a workspace with mixed `requires-python`
requirements, resolution will _never_ succeed, since we'll use the union
as the `requires-python` bound (i.e., take the lowest value), and fail
when we see the package that only supports some more narrow range.
This PR modifies the behavior to take the intersection (i.e., the
highest value), so if you have one package that supports Python 3.12 and
later, and another that supports Python 3.8 and later, we lock for
Python 3.12. If you try to sync or run with Python 3.8, we raise an
error, since the lockfile will be incompatible with that request.
Konsti has a write-up in https://github.com/astral-sh/uv/issues/5594
that outlines what could be a longer-term strategy.
Closes https://github.com/astral-sh/uv/issues/5578.
