Bump celery from 4.3.0 to 4.4.2

Bumps [celery](https://github.com/celery/celery) from 4.3.0 to 4.4.2.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog.rst)
- [Commits](celery/celery@v4.3.0...4.4.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -1,6 +1,6 @@
 amqp==2.5.2
 boto==2.49.0
-celery==4.3.0
+celery==4.4.2
 # click==7.0
 cryptography==2.7
 dj-database-url==0.5.0