Add GitHub actions

[sabinem]

This PR is about adding pytest and formatting check as github actions.

• Python versions 3.8 up to 3.11 are considered
• Poetry is used to install the dependencies
• black is used to check formatting of the python files

Additional changes:

• corrects the formatting of the python code so that it passes black
• changes the repo url for this repo in the tests to the github url instead of providing the local path
• adds a testconfig to avoid an ImportPathMismatchError raised by pytest, see Test modules with same name in different directories, all folders and parents containing __init__.py, raising _pytest.pathlib.ImportPathMismatchError. pytest-dev/pytest#10119

[cmdoret]

Looks great ! Thanks :)