Remove dependency on distutils, fixes #657

[robtaylor]

Replace using find_executable from distutils with which from shutil

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.24%. Comparing base (fe8fc8d) to head (9865124).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
doorstop/core/editor.py	50.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #664      +/-   ##
===========================================
- Coverage    96.32%   96.24%   -0.08%     
===========================================
  Files           41       41              
  Lines         5218     5220       +2     
  Branches       976     1226     +250     
===========================================
- Hits          5026     5024       -2     
- Misses         120      124       +4     
  Partials        72       72              

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