Misc/NEWS.d/next/Core and Builtins/2020-11-01-23-34-56.bpo-42233.zOSzja.rst

Allow ``GenericAlias`` objects to use :ref:`union type expressions <types-union>`.
This allows expressions like ``list[int] | dict[float, str]`` where previously a
``TypeError`` would have been thrown.  This also fixes union type expressions 
not de-duplicating ``GenericAlias`` objects.  (Contributed by Ken Jin in 
:issue:`42233`.)