Migrate from travis-ci to github actions

[phydev]

travis-ci is not working due to some change of policies regarding free plans. Therefore we should implement a github actions workflow to test, build and deploy.

[phydev]

Unit tests are now working with GitHub Actions, but coverage is not working because it doesn't identify trajpy as a package when importing it inside tests.

[phydev]

Readthedocs needs to be deployed with actions also:
https://github.com/readthedocs/actions