Update celery requirements to its latest version

[humitos]

Upgrading celery requires to upgrade their dependencies as
well (kombu, redis). By using their latest version, it could help with
the random disconnection of the workers that we suffer sometimes.

After upgrading them all, the errors we have found when we pinned the
versions they all disappeared.

Closes #4643

[stsewd]

Did you test with ALWAYS_EAGER = True? Core team should be using it to False, but new contributors will hit an error.

Also, this will closes #4643

[humitos]

Did you test with ALWAYS_EAGER = True?

Yes. All tests are ran with ALWAYS_EAGER = True.

[stsewd]

Yes. All tests are ran with ALWAYS_EAGER = True.

The error happens when triggering a build, I don't think it was detected inside the tests.

[humitos]

Confirm. Triggering a build does work.

[stsewd]

So, the issue on celery is still open celery/celery#5330

But I think we don't use this anymore

readthedocs.org/readthedocs/core/utils/__init__.py

Line 27 in 54397a0

def broadcast(type, task, args, kwargs=None, callback=None): # pylint: disable=redefined-builtin

That's why we don't hit this issue anymore.

[ericholscher]

I'm thinking we might still hit this in production, but I'm 👍 on trying to upgrade it, because falling super far behind on versions isn't good.

[ericholscher]

Let's merge this after our deploy today, so we have time to test it out in dev across different environments before we ship it to prod over the holidays.