Fix implementation dependent assertion in test_plistlib. (GH-4813) (#4815)

It is failed with an advanced optimizer.
(cherry picked from commit 0e069a15)

Lib/test/test_plistlib.py

@@ -321,7 +321,8 @@ class TestPlistlib(unittest.TestCase):
                     'second': [1, 2],
                     'third': [3, 4],
                 })
-                self.assertIsNot(pl2['first'], pl2['second'])
+                if fmt != plistlib.FMT_BINARY:
+                    self.assertIsNot(pl2['first'], pl2['second'])
 
     def test_list_members(self):
         pl = {