Evaluate Celery 4.3.0

Celery 4.3.0 has been released https://github.com/celery/celery/releases and should be reviewed for important bug fixes that might impact gwcelery.

If there are no important bug fixes then a low-priority task is to re-evaluate whether gwcelery can be upgraded to Python 3.7 based on this update to make sure that option is available when needed.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information