Add pylint and flake8 tests to GitLab CI

Add pylint and flake8 tests to GitLab CI

Our python code didn't adhere to any coding standard.  In this commit, we add
flame8 (https://pypi.org/project/flake8/), and pylint (https://www.pylint.org/).

There's couple of exceptions:

- ans.py scripts are not checked, nor fixed as part of this MR
- pylint's missing-*-docstring and duplicate-code checks have
  been disabled via .pylintrc

Both exceptions should be removed in due time.

(cherry picked from commit ee534592e3c5ecd9f0d6c115c43b680baa692ed3)