[deps] Update celery to version 5

[nemesifier]

Better to address openwisp/openwisp-controller#325 first.

Celery 5 should brings some breaking changes.

We should ensure the command to launch celery that is indicated in the README works and if not we need to update it.
Other than ensuring all tests pass, it's advised to do some manual testing running celery in the development environment, trigger the celery tasks and ensure they work.

[nemesifier]

There's no need to do anything here because it has been handled in openwisp-controller.