bpo-32996: Documentation fix-up. (GH-16646)

PR GH-4906 changed the typing.Generic class hierarchy, leaving an
outdated comment in the library reference. User-defined Generic ABCs now
must get a abc.ABCMeta metaclass from something other than typing.Generic
inheritance.
(cherry picked from commit d47f0dd)

Co-authored-by: M. Eric Irrgang <mei2n@virginia.edu>

Author: miss-islington

Doc/library/typing.rst

@@ -306,9 +306,10 @@ User defined generic type aliases are also supported. Examples::
    def inproduct(v: Vec[T]) -> T: # Same as Iterable[Tuple[T, T]]
        return sum(x*y for x, y in v)
 
-The metaclass used by :class:`Generic` is a subclass of :class:`abc.ABCMeta`.
-A generic class can be an ABC by including abstract methods or properties,
-and generic classes can also have ABCs as base classes without a metaclass
+.. versionchanged:: 3.7
+    :class:`Generic` no longer has a custom metaclass.
+
+A user-defined generic class can have ABCs as base classes without a metaclass
 conflict. Generic metaclasses are not supported. The outcome of parameterizing
 generics is cached, and most types in the typing module are hashable and
 comparable for equality.