[3.10] bpo-44524: Fix cryptic TypeError message when trying to subclass special forms in typing (GH-27710)

[miss-islington]

This was a Python 3.9 regression.
(cherry picked from commit a3a4d20)

Co-authored-by: Yurii Karabas [email protected]

https://bugs.python.org/issue44524

[miss-islington]

@uriyyo and @ambv: Status check is done, and it's a success ✅ .

[miss-islington]

@uriyyo and @ambv: Status check is done, and it's a success ✅ .

[miss-islington]

@uriyyo and @ambv: Status check is done, and it's a success ✅ .

[ambv]

@pablogsal, this is only a TypeError message improvement.

Before:

>>> class C(Union[int, str]): ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases

After:

>>> class C(Union[int, str]): ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cannot subclass <class 'typing._SpecialForm'>

The "after" was the situation before Python 3.9.0.