Fix mypy violations (for mypy-0.740)

[tk0miya]

Feature or Bugfix

• Refactoring

[codecov]

Codecov Report

Merging #6747 into 2.2.1 will decrease coverage by 0.02%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##            2.2.1    #6747      +/-   ##
==========================================
- Coverage   83.78%   83.75%   -0.03%     
==========================================
  Files         269      271       +2     
  Lines       40981    41015      +34     
  Branches     5999     6002       +3     
==========================================
+ Hits        34336    34353      +17     
- Misses       5321     5335      +14     
- Partials     1324     1327       +3

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
sphinx/util/pycompat.py	95.12% <0%> (ø)	⬆️
sphinx/util/docutils.py	91.76% <100%> (ø)	⬆️
sphinx/domains/cpp.py	82.91% <100%> (ø)	⬆️
sphinx/util/matching.py	100% <100%> (ø)	⬆️
sphinx/__init__.py	73.91% <0%> (ø)
sphinx/make_mode.py	0% <0%> (ø)

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