Add more tests

Author: vstinner

Lib/test/test_copy.py

@@ -439,6 +439,13 @@ def test_deepcopy_frozendict(self):
         self.assertEqual(y.keys(), x.keys())
         self.assertIsNot(x, y)
         self.assertIsNot(x["foo"], y["foo"])
+        self.assertIs(y['foo'][0], y)
+
+        x = frozendict(foo=[])
+        x['foo'].append(x)
+        x = x['foo']
+        y = copy.deepcopy(x)
+        self.assertIs(y[0]['foo'], y)
 
     @support.skip_emscripten_stack_overflow()
     @support.skip_wasi_stack_overflow()