Upload coverage from GHA

[hugovk]

Subset of #479.

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We already have tests on GitHub Actions, but are missing sending coverage to Codecov. Earlier it was tricky to do, but Codecov now have an action for this; added here.

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This also fixes a pytest warning:

=============================== warnings summary ===============================
.tox/py37/lib/python3.7/site-packages/_pytest/config/__init__.py:1230
  /home/travis/build/hugovk/tablib/.tox/py37/lib/python3.7/site-packages/_pytest/config/__init__.py:1230: PytestConfigWarning: Unknown config option: python_paths
  
    self._warn_or_fail_if_strict("Unknown config option: {}\n".format(key))
-- Docs: https://docs.pytest.org/en/stable/warnings.html

[codecov]

Codecov Report

Merging #480 (f91f369) into master (615e308) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #480   +/-   ##
=======================================
  Coverage   90.66%   90.66%           
=======================================
  Files          28       28           
  Lines        2613     2613           
=======================================
  Hits         2369     2369           
  Misses        244      244           

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