Fix #5492: sphinx-build fails to build docs w/ Python < 3.5.2

[tk0miya]

Feature or Bugfix

• Bugfix

Purpose

• refs: Fail to build docs w/ Python < 3.5.2 #5492

[codecov]

Codecov Report

Merging #5510 into 1.8 will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              1.8    #5510      +/-   ##
==========================================
+ Coverage   82.07%   82.07%   +<.01%     
==========================================
  Files         300      300              
  Lines       40026    40025       -1     
  Branches     6182     6182              
==========================================
  Hits        32850    32850              
  Misses       5799     5799              
+ Partials     1377     1376       -1

Impacted Files	Coverage Δ
sphinx/transforms/post_transforms/compat.py	63.41% <100%> (+1.5%)	⬆️

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