Add GitHub Actions CI

[eumiro]

First let's see whether it runs …

[eumiro]

Looks like it works. The results are visible at https://github.com/eumiro/eve/actions until you merge this.

Please edit the list of MongoDB and Redis versions you want to use in the .github/workflows/ci.yml file, otherwise the number of CI runs would be quite high and with little added value.

[nicolaiarocci]

rebased and merged, thank you.

[nicolaiarocci]

@eumiro two things:

• Docs don't have the GH Action CI badge yet;
• Once the above is done, I think we are ready to drop travis

[eumiro]

What do you mean by the docs missing the GHA CI badge? There's a badge in the README.rst file.

[nicolaiarocci]

if you go to python-eve.org you get a different file rendered on the homepage (https://github.com/pyeve/eve/blob/master/docs/index.rst)