Track supported Python range in lockfile (#4065)
## Summary
This PR adds the `Requires-Python` range to the user's lockfile. This
will enable us to validate it when installing.
For now, we repeat the `Requires-Python` back to the user;
alternatively, though, we could detect the supported Python range
automatically.
See: https://github.com/astral-sh/uv/issues/4052
