build: Add a default build command.

[feanil]

Python semantic release does not build the project by default, just
tags it. So if we want to publish the project to PyPI we need to add
the build command.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.23%. Comparing base (53c8f3f) to head (f6d8f3a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #20   +/-   ##
=======================================
  Coverage   88.23%   88.23%           
=======================================
  Files          13       13           
  Lines         408      408           
  Branches       16       16           
=======================================
  Hits          360      360           
  Misses         40       40           
  Partials        8        8           

Flag	Coverage Δ
unittests	88.23% <ø> (ø)

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