Ok, discussing with Quentin, it's due to the pre-commit-hook we have installed which refuses to let the user commits if there are some pep8 violations. And, as the module is imported but in appearance not used, for flake8, it's a fail.

We need to find a way to improve on this (or not).

I know it's possible to have some setup file for flake8 to ignore some errors (this one is F401).
But ignoring this error is too much - http://flake8.readthedocs.io/en/latest/warnings.html#error-codes.
We only want to ignore it in that particular case.

Also, i found this which relates to this http://stackoverflow.com/questions/31079047/python-pep8-class-in-init-imported-but-not-used. Following the proposed solution, this'd give something like:

from .director import ArchiverDirector
from .worker import ArchiverWorker
from .copier import ArchiverCopier

__all__ = ['ArchiverDirector', 'ArchiverWorker', 'ArchiverCopier']

I don't know if this is reasonable or not but it works.
As pros, this has the merit to force us to be explicit about what's public...
As cons, it's somehow redundant and instead of 18 characters, we have 66...

@olasd @zack What do you think?