Fork when minimum Python version increases (#8628)

## Summary

This is a re-implementation of
https://github.com/astral-sh/uv/pull/4712, though is now seemingly much
simpler. This issue keeps coming up, and users have a workaround with
`tool.uv.environments`, but it's really a bug in the resolver.

Closes https://github.com/astral-sh/uv/issues/4668.

crates/uv-resolver/src/python_requirement.rs

@@ -71,6 +71,12 @@ impl PythonRequirement {
         })
     }
 
+    /// Returns `true` if the minimum version of Python required by the target is greater than the
+    /// installed version.
+    pub fn raises(&self, target: &RequiresPythonRange) -> bool {
+        target.lower() > self.target.range().lower()
+    }
+
     /// Return the exact version of Python.
     pub fn exact(&self) -> &Version {
         &self.exact