Add code quality job to identity code quality issues (unused imports, etc)

In working on Remove custom kaiser window implementation (!4 - closed) I discovered a few unused imports in python/FIRtools.py. If I run flake8 --select=F401 on the whole project, I find 294 (other) unused imports. It would be great to add a CI/CD pipeline job to automatically run flake8 to discover this sort of issue so that these small issues can be identified and removed easily, or at least not re-introduced in new code.

I'm happy to propose a MR for this - sorry for sticking my nose (unasked) in your code, feel free to tell me to go away.

mentioned in merge request !5 (merged)

@duncanmmacleod Any project should be happy to receive this kind of feedback/review. Thank you.

mentioned in commit 4a7c2e80

closed with merge request !5 (merged)

Thank you for this Duncan! I've merged the relevant merge request.

moved to gstlal-calibration-reduced-storage#18