Bump celery from 4.2.1 to 4.3.0

Bumps [celery](https://github.com/celery/celery) from 4.2.1 to 4.3.0.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog)
- [Commits](https://github.com/celery/celery/compare/v4.2.1...v4.3.0

)

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

requirements.txt

@@ -14,7 +14,7 @@ sentry-sdk==0.12.3
 
 # Production dependencies
 boto3==1.9.245
-celery==4.2.1
+celery==4.3.0
 dj-database-url==0.5.0
 django-anymail==6.0.1
 django-basic-auth-ip-whitelist==0.2.1