Unittest workflow fix

[maskarb]

For whatever reason, a workflow was using python2 to run the python3.6 unit tests: https://github.com/project-koku/koku/runs/850175190?check_suite_focus=true 🤨

This PR:

• specifies to install pipenv using python3
• specifies the branch in the cache key <- this ensures that the dependencies are installed when the branch is first pushed to github

[codecov]

Codecov Report

Merging #2250 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2250     +/-   ##
========================================
+ Coverage    94.4%   94.5%   +0.1%     
========================================
  Files         232     232             
  Lines       16264   16442    +178     
  Branches     1826    1826             
========================================
+ Hits        15355   15533    +178     
+ Misses        581     580      -1     
- Partials      328     329      +1