Fix problem where `python` may not be available as python3 default app

[hogepodge]

python3 isn't always available as python. Chainging system call to python3 to be more portable.

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@d7c7352). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##             master    #261   +/-   ##
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  Coverage          ?   2.15%           
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  Files             ?       8           
  Lines             ?     835           
  Branches          ?       0           
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  Hits              ?      18           
  Misses            ?     817           
  Partials          ?       0           

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