Prioritize forks based on Python narrowing (#5642)

## Summary

First part of: https://github.com/astral-sh/uv/issues/4926. We should
solve forks that _don't_ expand the world of supported versions (e.g.,
`python_version >= '3.11'` enables us to select new packages, since we
narrow the supported version range).

crates/uv-resolver/src/requires_python.rs

@@ -350,6 +350,12 @@ impl<'de> serde::Deserialize<'de> for RequiresPython {
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
 pub struct RequiresPythonBound(Bound<Version>);
 
+impl Default for RequiresPythonBound {
+    fn default() -> Self {
+        Self(Bound::Unbounded)
+    }
+}
+
 impl RequiresPythonBound {
     pub fn new(bound: Bound<Version>) -> Self {
         Self(match bound {