Always narrow markers by Python version (#6076)

## Summary

Using https://github.com/astral-sh/uv/issues/6064 as a motivating
example: at present, on main, we're not properly propagating the
`Requires-Python` simplifications. In that case, for example, we end up
solving for a branch with `python_version < 3.11`, and a branch `>=
3.11`, even though `Requires-Python` is `>=3.11`. Later, when we get to
the graph, we apply version simplification based on `Requires-Python`,
which causes us to _remove_ the `python_version < 3.11` markers
entirely, leaving us with duplicate dependencies for `pylint`.

This PR instead tries to ensure that we always apply this narrowing to
requirements and forks, so that we don't need to apply the same
simplification when constructing the graph at all.

Closes https://github.com/astral-sh/uv/issues/6064.

Closes #6059.

crates/uv-resolver/src/python_requirement.rs

@@ -1,5 +1,4 @@
 use pep440_rs::{Version, VersionSpecifiers};
-use pep508_rs::MarkerTree;
 use uv_python::{Interpreter, PythonVersion};
 
 use crate::{RequiresPython, RequiresPythonBound};
@@ -70,17 +69,6 @@ impl PythonRequirement {
     pub fn target(&self) -> Option<&PythonTarget> {
         self.target.as_ref()
     }
-
-    /// Return a [`MarkerTree`] representing the Python requirement.
-    ///
-    /// See: [`RequiresPython::to_marker_tree`]
-    pub fn to_marker_tree(&self) -> Option<MarkerTree> {
-        if let Some(PythonTarget::RequiresPython(requires_python)) = self.target.as_ref() {
-            Some(requires_python.to_marker_tree())
-        } else {
-            None
-        }
-    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]