Adding basic events to celery Batches

[Anghille]

Following preceding works (not updated since), I do this PR to add the following modifications

• Add signals.task_received to tests
• Add events task-received, task-succeded and task-started
• Format to follow black /flake8 formatting

I intend (with your help if possible) to implement at least task-failed and task-retried events as well.

[clokep]

Following preceding works (not updated since)

I think this is based on #34?

[clokep]

I think this needs a rework -- the tests don't seem to quite make sense to the code changes. If you're still interested in working on it, let me know and we can reopen.

[Anghille]

I might take a look at some point, but I managed to do a workaround using an uglier solution for now (that is outside of this repo scope). It won't stand longterm but works for now. Thanks :)