Narrow `requires-python` requirement in resolver forks (#4707)
## Summary
Given:
```text
numpy >=1.26 ; python_version >= '3.9'
numpy <1.26 ; python_version < '3.9'
```
When resolving for Python 3.8, we need to narrow the `requires-python`
requirement in the top branch of the fork, because `numpy >=1.26` all
require Python 3.9 or later -- but we know (in that branch) that we only
need to _solve_ for Python 3.9 or later.
Closes https://github.com/astral-sh/uv/issues/4669.
