Drop django-celery; apparently we should just use celery now.

2 jobs for master in 2 minutes and 34 seconds
Status Job ID Name Coverage
  Test
failed #183628
test

00:02:34

1.0%
 
  Deploy
skipped #183629
allowed to fail manual
deploy
 
Name Stage Failure
failed
test Test
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
File "/builds/reproducible-builds/try.diffoscope.org/trydiffoscope/settings/__init__.py", line 3, in <module>
from .defaults import *
File "/builds/reproducible-builds/try.diffoscope.org/trydiffoscope/settings/defaults/__init__.py", line 6, in <module>
from celery.schedules import crontab
File "/builds/reproducible-builds/try.diffoscope.org/trydiffoscope/celery.py", line 5, in <module>
from celery import Celery
ImportError: cannot import name 'Celery'
ERROR: Job failed: exit code 1