Use Python 3 for the macOS build.

[mblayman]

To accept your contribution, please ensure that the checklist below is complete.

• Is your name/identity in the AUTHORS file?
• Does the code change (if the PR contains code) have 100% test coverage?
• Is CI passing all quality and testing checks?

[codecov-io]

Codecov Report

Merging #93 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #93   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         565    565           
=====================================
  Hits          565    565

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