Support authenticating using Redis usernames

[espadrine]

Redis added the AUTH command with only a password at first, and that is what the current Flower code assumes.
Redis added usernames in the AUTH command later, and added support for them in the redis-py library.

In this patch, we add support for usernames in Flower.

Note that while the latest release of Kombu allows many versions of the redis-py package, to support usernames, you need a version that includes the commit linked above. We tested redis-py v4.5.4.

[mher]

This change is not backwards compatible

[espadrine]

Thanks for discussing it!

What backwards incompatibility does it trigger?

[espadrine]

I added tests, in case what you were hinting at was that we want to detect backwards incompatibility on it.

I also tested the following:

1. Launched docker compose up;
2. Ran docker compose run flower celery -A tasks call tasks.add --args '[1, 1]';
3. Checked on http://localhost:5555/ that the task was successful.

The default docker-compose.yml configuration does not use a Redis username, which makes me believe there is no backwards incompatibility, but of course there could be something I overlooked.

(As an aside, tests/call-tasks.sh has commands with outdated positional parameters which are rejected by celery on the project Dockerfile. I can change that if you wish.)