Use python3 for black to support py>=3.6

[villebro]

CATEGORY

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• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

I'm currently using Python 3.7 without python3.6 in my env. Just calling python3 should be sufficient.

TEST PLAN

Tested to work with python 3.7.3

REVIEWERS

@john-bodley

[codecov-io]

Codecov Report

Merging #7964 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7964   +/-   ##
=======================================
  Coverage   65.61%   65.61%           
=======================================
  Files         469      469           
  Lines       22381    22381           
  Branches     2432     2432           
=======================================
  Hits        14686    14686           
  Misses       7574     7574           
  Partials      121      121

---

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[codecov-io]

Codecov Report

Merging #7964 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7964   +/-   ##
=======================================
  Coverage   65.61%   65.61%           
=======================================
  Files         469      469           
  Lines       22381    22381           
  Branches     2432     2432           
=======================================
  Hits        14686    14686           
  Misses       7574     7574           
  Partials      121      121

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a09f258...9ad2c89. Read the comment docs.