Apply suggestions from code review

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Author: brianschubert

src/typing_extensions.py

@@ -1986,6 +1986,8 @@ class _ConcatenateGenericAlias(list):
         __class__ = typing._GenericAlias
 
         def __init__(self, origin, args):
+            # Cannot use `super().__init__` here because of the `__class__` assignment
+            # in the class body (https://github.com/python/typing_extensions/issues/661)
             list.__init__(self, args)
             self.__origin__ = origin
             self.__args__ = args
@@ -2545,6 +2547,9 @@ def __typing_is_unpacked_typevartuple__(self):
         def __getitem__(self, args):
             if self.__typing_is_unpacked_typevartuple__:
                 return args
+            # Cannot use `super().__getitem__` here because of the `__class__` assignment
+            # in the class body on Python <=3.11
+            # (https://github.com/python/typing_extensions/issues/661)
             return typing._GenericAlias.__getitem__(self, args)
 
     @_UnpackSpecialForm