Update Celery to v4.4.0

[thinkh]

Currently we are using Celery 4.2.2 with Python 3.7. The most recent release (as of today) is Celery 4.4.0. At the moment we could not switch to that release, as it requires redis>=3.2.0 and is in conflict with our phovea_data_redis redis~=2.10.6. Hence, we must update the redis version in phovea_data_redis first, and then update the celery version here.