Add first Github Workflow

[willianpaixao]

Github Actions was introduced a couple of years ago and it's now the de-facto tool for building, testing, and publishing Open Souce projects hosted in Github.

This PR introduces a first and introductory pipeline to run all tests upon every push and pull PR creation.

Some important notes:

• I wanted to add support to Python 3.9, but there's an MYPY error and I wanted this pipeline to pass. Maybe after the issue is fixed (in another PR), 3.9 can be added.
• A also added configuration to spin the tests in Linux, macOS, and Windows runners. That's a feature not implemented in the current pipeline.
• When Python 3.5 was deprecated one year ago and Github Actions no longer has runners for it. So I had to leave it behind. Consider officially drop support to it.
• Note that this pipeline only runs the tox tests. If accepted, another workflow to build documentation and publish artifacts can be made.

[kclowes]

Thank you for the PR, and sorry it didn't get looked at earlier. We are going to keep our current workflow with CircleCI for now, but thanks for taking initiative. In the future, you might consider opening an issue first to make sure the feature you implement is something we'd consider pulling in.