Splits tox into multiple jobs

[billbrod]

With this PR, we now have three possible tox environments: the default (run pytest), fix (runs black and isort commands that edit src/ and some of docs), and check (just check those directories and raise error).

Then additionally made changes to ci.yml:

• adds new check job, which just runs the check tox environment.
• remove pip install -e .[dev] during set up of tox (because tox will install all requirements into its own
• final check job now requires all of tests, prevent_docs_absolute_links, and checkenvironments)

Thus our CI should be slightly faster (as we're no longer installing unnecessary packages and only running black+isort+flake8 with one OS / python version, instead of all)

I think that by editing envlist at the top of the tox.ini, we can make it so the default tox action is to run the test with a single python version and also run fix, do we want that?

Finally, CONTRIBUTING still needs to be updated. if this all looks good to you, I can go ahead and update it to reflect these changes.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.09%. Comparing base (ca25470) to head (64ea184).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #250   +/-   ##
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  Coverage   97.09%   97.09%           
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  Files          20       20           
  Lines        1857     1857           
=======================================
  Hits         1803     1803           
  Misses         54       54           

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[billbrod]

Most recent pushes:

• Update CONTRIBUTING to describe recent changes to tox
• adds parallelization for pytest, using pytest-xdist
• change tox's default behavior to run test with one python version and fix

[BalzaniEdoardo]

this is great! thanks Billy! I would keep the toxenv as is now, but no strong preference.