Introduce CI linter

[aleksslitvinovs]

Py-TestUI code is quite inconsistent and I think enforcing consistent code guidelines is quite beneficial.

I am suggesting adding a CI job for every commit to run some kind of Python linter, e.g., Pylint. This would also require defining what code style (black, autopep8, isort, YAPF) is used in the code.

[alvarolaserna]

this is in progress already