Error expiring plans with Celery 5.0.0

[PetrDlouhy]

I have got following import error with Celery 5.0.0:

  File "/app/blenderhub/apps/accounts/management/commands/expire_plans.py", line 2, in <module>
    from plans import tasks
  File "/app/src/django-plans/plans/tasks.py", line 4, in <module>
    from celery.task.base import periodic_task
ModuleNotFoundError: No module named 'celery.task'

Also, the Celery dependency should be optional, as django-plans can now be used with management command instead of Celery.