Move CI to GitHub Actions

[mtazzari]

To Fix #183 move the Travis CI workflow to GitHub Actions. Requirements:

• [x] do not rely anymore from Travis CI
• [x] run unit tests for multiple Python versions
• [ ] build and deploy docs -> Moved to dedicated PR won't do in this PR.

Differences between the Travis CI workflow and the new GitHub Actions workflow:

• Python 2.7: as of January 2021 Python 2.7 has been discontinued (e.g., conda forge does not build galario for Python 2.7 anymore). In this new workflow I stop testing for Python 2.7.
• Docs: the Travis CI steps to build and deploy docs on github pages relied on the doctr tool, which only works in Travis. It is recommended to find another way to deploy documentation. #186 suggests to move them to readthedocs.org. For that, it is better to consistently migrate the whole docs in a dedicated PR.

[codecov[bot]]

Codecov Report

Merging #184 (c90c41e) into master (566bfb6) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files           5        5           
  Lines         496      496           
=======================================
  Hits          486      486           
  Misses         10       10           

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[mtazzari]

I've opened a topic on the codecov forum: https://community.codecov.io/t/threshold-parameter-not-working/2623 If I don't get an answer by Monday I'll just remove codecov from the project: although useful, if it is prone to these kind of glitches (fails for 0.04% decrease in coverage), it can be discarded.

[mtazzari]

Ready to merge