Switch CI to GitHub actions

[maxfischer2781]

This PR switches the CI from Travis to GitHub actions.

Closes actions/toolkit#176.

[maxfischer2781]

After fixing up the test setup for PyPy3, it looks like TARDIS almost supports it. There appears to be an issue with handling SQLite3.

_sqlite3.OperationalError: database is locked

[codecov-io]

Codecov Report

Merging #178 (28e2457) into master (d989653) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     actions/toolkit#178   +/-   ##
=======================================
  Coverage   99.54%   99.54%           
=======================================
  Files          43       43           
  Lines        1769     1769           
=======================================
  Hits         1761     1761           
  Misses          8        8           

Impacted Files	Coverage Δ
setup.py	100.00% <ø> (ø)

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[maxfischer2781]

I've removed pypy3 for the time being, since the reporting of ontinue-on-error is intransparent. See also actions/runner#2347

[maxfischer2781]

Deferred the PyPy3 issue. See actions/toolkit#179.

[eileen-kuehn]

Awesome, even the badges have the correct links :)

[giffels]

Thanks a lot of your awesome work. I have only one little change to request. Could you remove the .travis.yaml as well, please?

[eileen-kuehn]

Thanks! Go for it 🥳

[giffels]

Could you apply the suggested change, please?

[giffels]

Thanks a lot for your awesome contribution. I am happy that CI has now moved to GitHub actions. Tests are very fast now!

[eileen-kuehn]

Still great ;)