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

[tk0miya]

Describe the bug

Recently, the implementation of typing.Annotated was changed on the HEAD of 3.10 (refs: python/cpython#27841). After this change, autodoc can't process annotations built by typing.Annotated well.

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

How to Reproduce

Python 3.10.0rc1+ (heads/3.10:779b9ae, Aug 29 2021, 13:48:15) [Clang 11.0.3 (clang-1103.0.32.59)] on darwin
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>>> from sphinx.util.typing import stringify
>>> from typing import Annotated
>>> assert stringify(Annotated[str, "foo", "bar"]) == "str"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
>>> stringify(Annotated[str, "foo", "bar"])
'Annotated'

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