sphinx.util.typing.stringify duplicates type arguments

[eric-wieser]

Describe the bug
Foo[int] is shown as Foo[int][int]

To Reproduce
Steps to reproduce the behavior:

In [6]: from typing import Generic, TypeVar

In [7]: T = TypeVar('T')

In [8]: class Foo(Generic[T]): pass

In [11]: sphinx.util.typing.stringify(Foo[int])
Out[11]: '__main__.Foo[int][int]'

Expected behavior
The output should be __main__.Foo[int]

Environment info

• OS: Windows 10
• Python version: 3.8.0
• Sphinx version: 3.0.2

[eric-wieser]

@tk0miya, I just assembed a patch for this.