This is not backward compatible with celery 3, so pin celery>=4

See http://docs.celeryproject.org/en/4.0/whatsnew-4.0.html for summary of changes

Change attributes from celery.app.config to lower case.
Drop setting serializer to json in README and tests since json is now the
default serializer.

I had issues with test_workflow_group() where AsyncResult().get on the *group*
task_id hang. It looks related to our test infrastructure and AFAIK there is no
known usage of this for cubes using cubicweb-celerytask.
I think we must dig into this bug but I already spent enough time on this, so
let's move forward and skip this part of the test.