## 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.
In the time before universal resolving, we would always pass a
`MarkerEnvironment`, and this environment would capture any relevant
`Requires-Python` specifier (including if `-p/--python` was provided on
the CLI).
But in universal resolution, we very specifically do not use a
`MarkerEnvironment` because we want to produce a resolution that is
compatible across potentially multiple environments. This in turn meant
that we lost `Requires-Python` filtering.
This PR adds it back. We do this by converting our `PythonRequirement`
into a `MarkerTree` that encodes the version specifiers in a
`Requires-Python` specifier. We then ask whether that `MarkerTree` is
disjoint with a dependency specification's `MarkerTree`. If it is, then
we know it's impossible for that dependency specification to every be
true, and we can completely ignore it.
## Summary
See the long comment inline. I think this is debatable but probably
right for now. The other options have their own problems, but there are
a few alternate ideas in the comment.
Closes https://github.com/astral-sh/uv/issues/4132.
konsti
Charlie Marsh
Andrew Gallant