Suppress arguments if all system defined TypeVars on py39

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• It seems the implementation of the typing module has changed since py3.9.0a6+.

• It causes the following error.

          sig = inspect.signature(f18)
  >       assert stringify_signature(sig) == '(self, arg1: Union[int, Tuple] = 10) -> List[Dict]'
  E       AssertionError: assert '(self, arg1:...Dict[KT, VT]]' == '(self, arg1:...-> List[Dict]'
  E         - (self, arg1: Union[int, Tuple] = 10) -> List[Dict]
  E         + (self, arg1: Union[int, Tuple] = 10) -> List[Dict[KT, VT]]
  E         ?                                                  +++++++ +
  

  https://travis-ci.org/github/sphinx-doc/sphinx/jobs/684582074#L1841-L1846

• This fixes them with checking arguments are TypeVars on typing module.

[tk0miya]

I need to merge this into 3.0.x branch...