YML file: CI Migration to Github Actions

[mvargas33]

Resolves #325

Developed a draft YML file that handles the migration from Travis CI to Github actions.

• Python 3.6, 3.7 and 3.8 are tested with sklearn=0.20.3 + skggm for compatibility. Python 3.9 doesn't work for some reason
• Python 3.6, 3.7, 3.8 and 3.9 are tested with latest versions of dependencies, with and without skggm
• Test reports are uploaded to Codecov, as before
• Flake8 always run in the end
• All tests run on the latest Ubuntu x64

Sorry for making too many commits, I could not find a way to debug this. But only one file is added and its automatically recognized by Guthub Actions.

This proposal is already working on my fork of metric_learning, at master branch.

[terrytangyuan]

Thanks! Just a couple of comments.

[perimosocordiae]

+1 to merge this after resolving @terrytangyuan's comments

[mvargas33]

All comments were resolved. All tests passed on my fork. File is ready to merge