Change package requirements.

Use "celery" instead of "django_celery" since:

  - This package does not explicitly require the Django integration
    features in "django_celery".
  - "django_celery" is not supported in recent versions of celery:
    celery/django-celery#597 (comment)

Also upgrade minimum Celery version from 3.2.0 to 4.4.0, since former
version does not support any currently support version of Django.

requirements.txt

@@ -1 +1 @@
-django_celery>=3.2.0
+celery>=4.4.0