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This is not backward compatible with celery 3, so pin celery>=4

See http://docs.celeryproject.org/en/4.0/whatsnew-4.0.html for summary of changes

Change attributes from celery.app.config to lower case.
Drop setting serializer to json in README and tests since json is now the
default serializer.

I had issues with test_workflow_group() where AsyncResult().get on the *group*
task_id hang. It looks related to our test infrastructure and AFAIK there is no
known usage of this for cubes using cubicweb-celerytask.
I think we must dig into this bug but I already spent enough time on this, so
let's move forward and skip this part of the test.

It was unmaintained and I don't want to maintain it.

Previously we handle monitoring celery task by listening to celery event bus
(celery.events.EventReceiver) that was not persistent. In this case we used a
dedicated routine (on_monitor_start) to synchronize non finished tasks, but
this wasn't working in case of an untracked task (eg. not started with
start_async_task). Also this was a single point of failure because it cannot
run in multiple instance without concurrency issues (events are sent to all
instances)

Now we use a redis queue where worker put task_id and task_name to be
synchronized and celery-monitor use brpop (https://redis.io/commands/brpop) to
process the queue.

We don't require CELERY_SEND_EVENTS to be enabled anymore (-E or --events in
worker options).
We require to add 'cw_celerytask_helpers.helpers' to CELERY_IMPORTS.

to explain a bit the example celeryconfig.py file.

Provides run_all_task() and get_tasks() methods instead.

This allow to make assertions before and after running tasks and/or introspect
task signatures.

Also, set CELERY_ALWAYS_EAGER and CELERY_EAGER_PROPAGATES_EXCEPTIONS to True by
default, this can be overriden in setUp() method.

Also add a task example with celery.signature

Adapters based API
Inspired from https://hg.logilab.org/users/david/cw-celery-logging-demo/