CI: Move travis -> github action

[tony]

Cherry-picked from #300

[codecov]

Codecov Report

Merging #301 into master will increase coverage by 3.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   76.73%   80.40%   +3.66%     
==========================================
  Files           5       12       +7     
  Lines         288      648     +360     
==========================================
+ Hits          221      521     +300     
- Misses         67      127      +60     

Impacted Files	Coverage Δ
vcspull/util.py	93.33% <0.00%> (-6.67%)	⬇️
tests/conftest.py	80.76% <0.00%> (ø)
tests/helpers.py	79.16% <0.00%> (ø)
tests/test_config.py	97.32% <0.00%> (ø)
tests/test_repo_object.py	72.44% <0.00%> (ø)
tests/test_cli.py	50.00% <0.00%> (ø)
tests/fixtures/example.py	100.00% <0.00%> (ø)
setup.py	0.00% <0.00%> (ø)

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