text: Title: Subscription is not triggered from background task
  Body: I am using a Celery periodic_task that triggers  a Subscription.
  
  ```
  from .schema import OnUpdateOrderStatus # subscription
  
  @periodic_task(run_every=crontab(minute='*/1'), name="completed_appointments")
  def completed_appointments():
  
          ...
  
          OnUpdateOrderStatus.update_order_status(order=order, customer_id=customer.pk)
  
  ```
  
  - [x] When I call OnUpdateOrderStatus from a Mutation or a Query on the same file it works
  
  - [x] when I call external methods in another module from this celery periodic_task it works too,
  
  anyone has tried to import a subscription class and call it from another module/file?
  
  Thank you
  