feat: add a new shell script to update CW flake8-ok-files.txt

It's a simple bash script that will update CubicWeb's infamous flake8-ok-files.txt

I first started to integrated that directly into CW itself but then realized it was way simpler to add it here and eventually use it on other projects if needed.

The idea is to run it like once per week (during the weekend for example) because this is something we really don't want to to put any time or energy on it but we still want to move in a direction were we can remove it and solidify the foundation that we already have and add files that we know are ok. Eventually we'll reach a point where we can ditch this.

I'm not sure am going to wait that much for a real review since this is more one of my pet peeve to trash one of my constant frustration than any major work 😅

As you can see there are quite a lot of stuff to add in this file:

And I'll have my small please of seeing it slowly growing in size.