Add Black formatting and use GitHub workflows

[DeD1rk]

Closes #385.
Closes #78.

This replaces Travis CI with GitHub actions, and adds formatting with Black.

It also adds a workflow for deploying MkDocs docs and publishing to PyPI upon publishing a GitHub release.

[DeD1rk]

The CI added here of course only runs on my fork, so you can't really see it in action (but you should be able to view the result on the fork).

[DeD1rk]

@jmaupetit in aae44fa I added a workflow for mkdocs (so it replaces #83) and publishing to PyPI too, but I can't really test that, and for PyPI it needs an API token as a secret, so you should consider whether you want to use that or not, and perhaps test it against TestPyPI (https://test.pypi.org).