Revert Celery to 4.4.6

It looks like 4.4.7 has some issues with Redis broker:

celery/celery#6285

{{cookiecutter.project_slug}}/requirements/base.txt

@@ -17,7 +17,7 @@ redis==3.5.3  # https://github.com/andymccurdy/redis-py
 hiredis==1.1.0  # https://github.com/redis/hiredis-py
 {%- endif %}
 {%- if cookiecutter.use_celery == "y" %}
-celery==4.4.7  # pyup: < 5.0  # https://github.com/celery/celery
+celery==4.4.6  # pyup: < 5.0,!=4.4.7  # https://github.com/celery/celery
 django-celery-beat==2.0.0  # https://github.com/celery/django-celery-beat
 {%- if cookiecutter.use_docker == 'y' %}
 flower==0.9.5  # https://github.com/mher/flower