Fix for bug #1109

Warning required when calling register() on an ABCMeta subclass.

Lib/test/test_abc.py

@@ -146,6 +146,13 @@ class TestABC(unittest.TestCase):
         C()
         self.assertEqual(B.counter, 1)
 
+    def test_error_on_subclass(self):
+        class A(abc.ABCMeta):
+            pass
+        class B:
+            pass
+        self.assertRaises(TypeError, A.register, B)
+
 
 def test_main():
     test_support.run_unittest(TestABC)