Stop automatically converting the amqp:// prefix to librabbitmq:// + refactored test

[Alireza2n]

Based on issue #4693 on celery repo, librabbitmq is no longer the recommended package, but the code in kombu does not follow this recommendation.
It's also been discussed in PR #5873.
This PR removes the code where kombu.Connection()'s __init__() method automatically converts the amqp:// prefix to librabbitmq://.
Also test_prefer_librabbitmq_over_amqp_when_available() was rewritten to test_stop_converting_amqp_to_librabbitmq_when_available().

[auvipy]

there is flake8 issue to address

[Alireza2n]

there is flake8 issue to address

Done. It was caused by an unused import.

[auvipy]

AttributeError: <module 'kombu.connection' from '/home/travis/build/celery/kombu/kombu/connection.py'> does not have the attribute 'supports_librabbitmq'

[thedrow]

@auvipy We're almost ready for Celery 5.
I think we should keep this for that release since this is a breaking change.

Removing the recommendation is a step in the right direction for Celery 4.x.

@Alireza2n What do you think?

[auvipy]

yup removing recommendation could be the first move

[Alireza2n]

@auvipy We're almost ready for Celery 5.
I think we should keep this for that release since this is a breaking change.

Removing the recommendation is a step in the right direction for Celery 4.x.

@Alireza2n What do you think?

@thedrow Hi!
Use of librabbitmq as an optimization recommendation is already removed from celery documentation.
I'm not sure if this is a breaking change, I'm hoping to get the test and flak8 fixed this weekend.

[thedrow]

Why isn't this a breaking change?
If people upgrade and they still use librabbitmq they still expect to use it afterwards.

[Alireza2n]

Why isn't this a breaking change?
If people upgrade and they still use librabbitmq they still expect to use it afterwards.

Well, the existing code switched automatically to librabbitmq, I've removed this behavior and it seems it does not break anything (at least in automatic tests), users still can manually specify their library I believe.

[thedrow]

Yes but it shouldn't switch without a major version bump.

[Alireza2n]

Yes but it shouldn't switch without a major version bump.

Got it.
I'll try to fix it, then you can decide on release and other details. :)