Add pypy in build pipeline

[argaen]

Closes #357

[codecov-io]

Codecov Report

Merging #359 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #359   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          11       11           
  Lines         897      897           
  Branches       96       96           
=======================================
  Hits          893      893           
  Misses          4        4

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