Fix #5306: autodoc: getargspec() raises NameError for invalid typehints

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: Some weird exceptions when building #5306

[codecov]

Codecov Report

Merging #5309 into 1.7 will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##              1.7    #5309      +/-   ##
==========================================
- Coverage   82.05%   82.04%   -0.01%     
==========================================
  Files         284      284              
  Lines       37929    37932       +3     
  Branches     5880     5880              
==========================================
  Hits        31121    31121              
- Misses       5495     5498       +3     
  Partials     1313     1313

Impacted Files	Coverage Δ
sphinx/ext/autodoc/inspector.py	8.41% <0%> (-0.25%)	⬇️

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