Resolve "(ci): add a lint job"

Description

A lint phase was added, which runs flake8.

Two expectations were added:

• Error E731 is skipped because I just don't like replacing a one-liner with 6 lines.
• Warning W503 is skipped because one have to choose between W504 or W503. Pick your poison... 😸

A fix was also made in the licence year replacement. When having a single old date, sheldon just replaced the year as it is, whereas it was supposed to create a range with the old date and the new date, i.e. 2009 should give 2009-2021 and not 2021. A unit-test has been added to avoid future regressions.

Closes #25 (closed)

How to test it?

Just watch the pipeline result.