Fix frozenset copy to return self (#2883)

Fix frozenset copy to return zelf if called on frozenset instance

Lib/test/test_set.py

@@ -724,8 +724,6 @@ class TestFrozenSet(TestJointOps, unittest.TestCase):
             results.add(hash(self.thetype(seq)))
         self.assertEqual(len(results), 1)
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_copy(self):
         dup = self.s.copy()
         self.assertEqual(id(self.s), id(dup))

vm/src/builtins/set.rs

@@ -652,9 +652,14 @@ impl PyFrozenSet {
     }
 
     #[pymethod]
-    fn copy(&self) -> Self {
-        Self {
-            inner: self.inner.copy(),
+    fn copy(zelf: PyRef<Self>, vm: &VirtualMachine) -> PyRef<Self> {
+        if zelf.class().is(&vm.ctx.types.frozenset_type) {
+            zelf
+        } else {
+            Self {
+                inner: zelf.inner.copy(),
+            }
+            .into_ref(vm)
         }
     }