Code formatting improvements

[jraddaoui]

A few enhancements suggested along the way:

• Add proper names to mocked functions on tests.
• Move static variables to class attributes in tests.
• Properly format doc-strings in the project.
• Use black for auto-formatting.

[jraddaoui]

Hi @stefanabreitwieser,

This were just some "minor" changes to improve the code quality of the project and the CI checks related to that. I've created #147 to add some notes about this in the contributing file alongside other improvements that are needed in there, but I think this one could be closed.