Fix handling of changes to `requires-python` (#14076)
When using `uv lock --upgrade-package=python` after changing
`requires-python`, it was possible to get into a state where the fork
markers produced corresponded to the empty set. This in turn resulted in
an empty lock file.
There was already some infrastructure in place that I think was perhaps
intended to handle this. In particular, `Lock::check_marker_coverage`
checks whether the fork markers have some overlap with the supported
environments (including the `requires-python`). But there were two
problems with this.
First is that in lock validation, this marker coverage check came
_after_ a path that returned `Preferable` (meaning that the fork markers
should be kept) when `--upgrade-package` was used. Second is that the
marker coverage check used the `requires-python` in the lock file and
_not_ the `requires-python` in the now updated `pyproject.toml`.
We attempt to solve this conundrum by slightly re-arranging lock file
validation and by explicitly checking whether the *new*
`requires-python` is disjoint from the fork markers in the lock file. If
it is, then we return `Versions` from lock file validation (indicating
that the fork markers should be dropped).
Fixes #13951
