Fix pylint issues

With pylint and flake8 linters introduced in #991 (closed), we are now aware of some problems in the python code.

This ticket is about addressing those deficiencies. See !588 (merged) for useful comments, especially regarding how it's done in Forge.

The goal of this ticket is to:

• fix reported problems or disable warnings to reach 10/10 score in pylint.
• tweak .pylint to require 10 score or the pipeline should fail

Also, it would be useful to add/update section about linter in the dev guide (once #786 (closed) is merged).