Revert "Update celery requirements to its latest version"

[agjohnson]

This reverts commit 5939bca.

We noticed a lot of strange issues, including Celery dying without an
error, using the upgraded packages and even 4.4.0. Reverting back to
something that works for now.

[agjohnson]

Hotfixed on relcorp only

[ericholscher]

Seemed ambitious that all the issues would be fixed given how many caveats we had in the requirements file :/ Not sure how to move forward with this upgrade though.

[benjaoming]

Are there issues tracking this now?

#6448 and #4643 are closed.

[humitos]

No. We only have this note in the code:

readthedocs.org/requirements/pip.txt

Lines 40 to 42 in 97124cf

	# Celery 4.2 is incompatible with our code
	# when ALWAYS_EAGER = True
	celery==4.1.1 # pyup: ignore

We rely a lot on Celery to make Read the Docs to work. Since it's hard to test locally all the possible scenarios we usually try to upgrade Celery only when needed or try to make a deploy that only involves upgrading Celery to be able to monitor it from close.

IIRC, when we deployed the latest version, workers where randomly disconnected or they stopped consuming tasks and start doing nothing. That's why we decided to revert this change.

I'm curious, why you are interested in upgrading Celery, @benjaoming ?

[benjaoming]

@humitos I'm running Read the Docs on Python 3.7 (didn't realize it was intended for 3.6). After a lot of setup, I realized that Celery was broken (the classical async syntax keyword issue) and now I'm running it with a local patch that upgrades Celery.

I'll let you know if I experience the same hang-ups :)