Test dict values iterator pickling with pickle.HIGHEST_PROTOCOL. (GH-9052)

Lib/test/test_dict.py

@@ -1056,7 +1056,7 @@ class DictTest(unittest.TestCase):
             self.assertEqual(dict(it), data)
 
     def test_valuesiterator_pickling(self):
-        for proto in range(pickle.HIGHEST_PROTOCOL):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             data = {1:"a", 2:"b", 3:"c"}
             # data.values() isn't picklable, only its iterator
             it = iter(data.values())