use exist_ok instead of explicit check for path exists when creating hash cache so it is multiprocess safe

[tclose]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

Avoid race-condition problem where two processes try to create the user hash cache directory at the same time, by switching to exists_ok=True kwarg of mkdir instead of explicit check that path exists beforehand

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.22%. Comparing base (811dc45) to head (db42246).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #735      +/-   ##
==========================================
- Coverage   84.22%   84.22%   -0.01%     
==========================================
  Files          25       25              
  Lines        5123     5121       -2     
  Branches     1449     1446       -3     
==========================================
- Hits         4315     4313       -2     
  Misses        802      802              
  Partials        6        6              

Flag	Coverage Δ
unittests	84.22% <100.00%> (-0.01%)	⬇️

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