Type annotations using typing._GenericAlias are not displayed well with the HEAD of 3.10

[tk0miya]

Describe the bug

Recently, the implementation of typing._GenericAlias was changed on the HEAD of 3.10 (refs: python/cpython#27614). After this change, autodoc can't process type aliases built by typing._GenericAlias well.

Note: this breaks our CI. So it should be fixed on 4.1.3.

How to Reproduce

Python 3.10.0rc1+ (heads/3.10:4e0147e, Aug 11 2021, 00:14:32) [Clang 11.0.3 (clang-1103.0.32.62)] on darwin
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>>> from sphinx.util.typing import stringify
>>> from typing import Optional
>>> assert stringify(Optional[int]) == "Optional[int]"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> stringify(Optional[int])
'Optional[int, None]'

Expected behavior

No response

Your project

N/A

Screenshots

No response

OS

Linux

Python version

HEAD of 3.10

Sphinx version

4.1.x

Sphinx extensions

No response

Extra tools

No response

Additional context

No response