Trim injected `python_version` marker to (major, minor) (#2395)
## Summary
Per [PEP 508](https://peps.python.org/pep-0508/), `python_version` is
just major and minor:
Right now, we're using the provided version directly, so if it's, e.g.,
`-p 3.11.8`, we'll inject the wrong marker. This was causing `pandas` to
omit `numpy` when `-p 3.11.8` was provided, since its markers look like:
```
Requires-Dist: numpy<2,>=1.22.4; python_version < "3.11"
Requires-Dist: numpy<2,>=1.23.2; python_version == "3.11"
Requires-Dist: numpy<2,>=1.26.0; python_version >= "3.12"
```
Closes https://github.com/astral-sh/uv/issues/2392.
