chore: Add new rules for flake8

[pnu-s]

Adding new linting rules for flake8 with some plugins:

• pep8-naming to check variable naming (already compliant)
• flake8-import-order to check import order (fixed the compliance issues)
• flake8-use-fstring to use f-strings everywhere (fixed the compliance issues)

For an easier review, better to check commit by commit :nerd_face:

Ideally I'd like to push these for all our repositories, but starting with ETIP first!