Add flake8 to CI to check code compliance with PEP8 coding style
Desired behavior is the following.
- Push to repository should be only successful if flake8 output does not complain.
- Merge request should be annotated with flake8 output, if any, upon submission, so that the contributor(s) can take notice and make necessary changes in case they didn't do that already.
Additionally, developers can use pylint to identify code smells during development on their local environments.