Draft: integrate black
Using "hg status --change $rev" on all topics listed by "hg heads" I've applied black on all files that weren't modified by any possible or existing MR, this way we won't have problem merging other MR if we merge this one.
This used the ugly -ok-files.txt pattern of flake8 but at least this is a way forward. Now 480 files are handled by black.
In addition I've also update flake8-ok-files.txt and we are now at 377 ok files for flake8.