Add code coverage reports and upload them.

[HexDecimal]

Tox told to install the package in development mode to make a few things work such as test coverage paths.

Tox now reports code coverage using pytest-cov. Closes #16

Using Codecov.io for no reason other than that I'm most familiar with it. You can see how it looks here. There might be more to setup for this, you might have to enable kvas-it/pytest-console-scripts on codecov.io. I'll add a badge for this later.

I could not get tox-gh-actions working as well as I wanted. I ended up removing it and using the py env instead. I can revert this if you want. Linters are being run on all versions of Python in the CI, Mypy benefits from this but it could be handled better. This is mostly due to my inexperience with Tox.

Tox linting changed to use the active version of Python.

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@c599713). Click here to learn what that means. The diff coverage is n/a.

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@@            Coverage Diff            @@
##             master      #68   +/-   ##
=========================================
  Coverage          ?   99.42%           
=========================================
  Files             ?        1           
  Lines             ?      174           
  Branches          ?        0           
=========================================
  Hits              ?      173           
  Misses            ?        1           
  Partials          ?        0