tests: celery 4 by default; handle django-celery

[blueyed]

Only install django-celery for Django versions where it is actually used. This also requires to use Celery<4 there then [1].

1: https://github.com/celery/django-celery/commit/4ed4067e94e141a37baa7b085906766fb162d676.

TODO:

• [ ] squash