Avoid retaining forks when `requires-python` range changes (#7624)
## Summary
If the `requires-python` bound expands, the space covered by
`resolution-markers` may no longer include all supported Python
versions. In such cases, we need to avoid reusing the forks (but we
_can_ reuse the preferred versions).
Closes https://github.com/astral-sh/uv/issues/7618.
