Treat Python version as a lower bound in --universal (#4597)

## Summary

Closes #4591.

crates/uv-cli/src/lib.rs

@@ -569,8 +569,12 @@ pub struct PipCompileArgs {
     pub python_platform: Option<TargetTriple>,
 
     /// Perform a universal resolution, attempting to generate a single `requirements.txt` output
-    /// file that is compatible with all operating systems, architectures and supported Python
-    /// versions.
+    /// file that is compatible with all operating systems, architectures, and Python
+    /// implementations.
+    ///
+    /// In universal mode, the current Python version (or user-provided `--python-version`) will be
+    /// treated as a lower bound. For example, `--universal --python-version 3.7` would produce a
+    /// universal resolution for Python 3.7 and later.
     #[arg(long, overrides_with("no_universal"))]
     pub universal: bool,
 

crates/uv-resolver/src/python_requirement.rs

@@ -74,12 +74,15 @@ impl PythonRequirement {
         let version_major_minor_only = Version::new(version.release().iter().take(2));
         let expr_python_version = MarkerExpression::Version {
             key: MarkerValueVersion::PythonVersion,
-            specifier: VersionSpecifier::from_version(Operator::Equal, version_major_minor_only)
-                .unwrap(),
+            specifier: VersionSpecifier::from_version(
+                Operator::GreaterThanEqual,
+                version_major_minor_only,
+            )
+            .unwrap(),
         };
         let expr_python_full_version = MarkerExpression::Version {
             key: MarkerValueVersion::PythonFullVersion,
-            specifier: VersionSpecifier::from_version(Operator::Equal, version).unwrap(),
+            specifier: VersionSpecifier::from_version(Operator::GreaterThanEqual, version).unwrap(),
         };
         MarkerTree::And(vec![
             MarkerTree::Expression(expr_python_version),