Add Python 3's pip among the packages to install on Ubuntu

[codecov[bot]]

Codecov Report

Merging #26 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   68.64%   68.64%           
=======================================
  Files          23       23           
  Lines         574      574           
=======================================
  Hits          394      394           
  Misses        180      180

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[hbmartin]

thanks!