Very minor changes to AsyncGenerator (#356)

Author: ilevkivskyi

src/test_typing.py

@@ -1844,6 +1844,7 @@ def athrow(self, typ, val=None, tb=None):
         self.assertIsInstance(instance, collections.AsyncGenerator)
         self.assertIsInstance(instance, collections.AsyncIterable)
         self.assertNotIsInstance(type(g), G)
+        self.assertNotIsInstance(g, G)
 
     def test_subclassing_subclasshook(self):
 

src/typing.py

@@ -1902,10 +1902,8 @@ def __new__(cls, *args, **kwds):
         return _generic_new(_G_base, cls, *args, **kwds)
 
 if hasattr(collections_abc, 'AsyncGenerator'):
-    _AG_base = collections_abc.AsyncGenerator
-
     class AsyncGenerator(AsyncIterator[T_co], Generic[T_co, T_contra],
-                         extra=_AG_base):
+                         extra=collections_abc.AsyncGenerator):
         __slots__ = ()
 
     __all__.append('AsyncGenerator')