Backmerge -r59233:59232

Guido said:
Please roll this back.  The error message you added is inappropriate
when the parameter to a legitimate register() call is omitted, e.g.

collections.Sequence.register()

Lib/abc.py

@@ -137,11 +137,8 @@ class ABCMeta(type):
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls
 
-    def register(cls, subclass=None):
+    def register(cls, subclass):
         """Register a virtual subclass of an ABC."""
-        if subclass is None:
-            raise TypeError("register() cannot be called on an ABCMeta "
-                "subclass, use class Example(metaclass=abc.ABCMeta) instead.")
         if not isinstance(cls, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):

Lib/test/test_abc.py

@@ -146,13 +146,6 @@ 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)