Move tests into a separate package

[Kentzo]

• Move configuration into setup.cfg
• Add integration with codecov (repo owner still need to enable it)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3cb7737). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #61   +/-   ##
=========================================
  Coverage          ?   86.89%           
=========================================
  Files             ?       19           
  Lines             ?     1907           
  Branches          ?      294           
=========================================
  Hits              ?     1657           
  Misses            ?      164           
  Partials          ?       86

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[llllllllll]

What is the point of all of this? What do we get out of it?

[Kentzo]

Codecov is a convenient way to see how much of the project is covered with tests.
Moving tests out of the main repo allows not to ship them.
Having config in one file is convenient.