Migrate to py3 style type annotation: sphinx.testing

[tk0miya]

Feature or Bugfix

• Refactoring

Purpose

• refs: Move to python3 style type annotation #6413

[codecov]

Codecov Report

Merging #6555 into 2.0 will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              2.0    #6555      +/-   ##
==========================================
+ Coverage   83.77%   83.78%   +<.01%     
==========================================
  Files         266      266              
  Lines       40806    40814       +8     
  Branches     5981     5981              
==========================================
+ Hits        34187    34195       +8     
  Misses       5296     5296              
  Partials     1323     1323

Impacted Files	Coverage Δ
sphinx/testing/path.py	88.05% <100%> (+0.55%)	⬆️
sphinx/testing/fixtures.py	85.58% <100%> (+0.26%)	⬆️
sphinx/testing/restructuredtext.py	100% <100%> (ø)	⬆️
sphinx/testing/util.py	91.89% <100%> (+0.07%)	⬆️

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