Fix tox config

[hugovk]

Put test and docs in their own toxenvs to fix test runs and re-enable coverage checks on PRs.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d0963c2). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #449   +/-   ##
=========================================
  Coverage          ?   90.39%           
=========================================
  Files             ?       28           
  Lines             ?     2583           
  Branches          ?        0           
=========================================
  Hits              ?     2335           
  Misses            ?      248           
  Partials          ?        0

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