Python version pins need to be combined with Requires-Python

[zanieb]

As briefly discussed at #4989 (comment)

When reading pins from .python-version files in workspaces, the Requires-Python information may need to be layered to resolve the correct Python interpreter e.g. with

• 3.10 and 3.12 on the PATH
• A project that requires >=3.11
• And a pin of cpython

We will currently resolve to 3.10 since ignore the project Python requirement entirely when there is a pin.

Instead, we should combine the PythonRequest::Implementation and the Requires-Python into a PythonRequest::ImplementationVersion(..., VersionRequest::Range(...)).

[zanieb]

This could get pretty complicated. We may want to think about the problem space a bit before implementing the suggestion.