Fix the error for directory separator

[liferoad]

#34364 reverted the PR.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.37%. Comparing base (3595a33) to head (ddbb675).

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #34390      +/-   ##
============================================
- Coverage     56.37%   56.37%   -0.01%     
  Complexity     3285     3285              
============================================
  Files          1174     1174              
  Lines        179353   179362       +9     
  Branches       3398     3398              
============================================
+ Hits         101119   101122       +3     
- Misses        74974    74980       +6     
  Partials       3260     3260              

Flag	Coverage Δ
python	81.30% <100.00%> (-0.01%)	⬇️

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[kennknowles]

Is it possible to actually cause this error, instead? Mocking makes the test possibly masking things if the presumed behavior of os.path.dirname is not quite right, or if it changes.