Implement Pylint rules for the Python codebase

[im-prakher]

The aim of this issue is to make the python codebase follow all standard rules by a linter. Right now, some warnings are ignored in the pylintrc file, make the codebase follow all these standard rules. This will make the YABA app better.

Actually, there were a lot of warnings for inconsistent-return-statements, missing-module-docstring and somehow Pylint action treats them as an error, not warning and so it fails the test for linting.

Originally posted by @im-prakher in #63 (comment)

[dlebauer]

@im-prakher is the goal here to implement the pylint checker, or to fix all of the errors and warnings?

[im-prakher]

It is to fix all the linting errors and warnings which are right now ignored by pylint