Test Windows, macOS and Linux on GitHub Actions

[hugovk]

We currently run the tests on Linux on Travis CI, for Python 3.6-3.8.

This PR runs the tests for Windows, macOS and Linux on GitHub Actions, for Python 3.5-3.7.

Python 3.8 is not yet available:

• https://github.com/actions/setup-python/issues/30

We could skip Linux tests here, but GitHub Actions gives 20 parallel jobs (compared to 5 with Travis) so it including these 3 jobs here won't slow it down.

These are the py{35,36,37,38}-tests from tox.ini.

The py37-{docs,lint} tests are not done here, they're done on Travis CI, but could be added here.

Example test run:

• https://github.com/hugovk/tablib/commit/e0e75ed43ca70622863d35bd37ef2d8bfea5d76b/checks?check_suite_id=272272605

[codecov[bot]]

Codecov Report

Merging #409 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #409   +/-   ##
=======================================
  Coverage   80.81%   80.81%           
=======================================
  Files          22       22           
  Lines        1647     1647           
=======================================
  Hits         1331     1331           
  Misses        316      316

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