Python 3.9 support & old dependencies

[marksweb]

Tests against python 3.9 are failing.

https://github.com/groveco/django-sql-explorer/runs/4761199838?check_suite_focus=true

It would appear to be related to third party dependencies of the optional packages. Specifically a package called anyjson which was last updated in 2012.

This appears to be linked to celery/kombu which also could do with being updated.

#437 mentions support for Celery 5, but that's quite different in it's approach to tasks. Perhaps celery 4 support would be the best first step.

[marksweb]

#461 has python 3.9 tests passing and is an initial look at testing against python 3.10