[AIRFLOW-3353] Upgrade redis client

[ashb]

Make sure you have checked all steps below.

Jira

https://issues.apache.org/jira/browse/AIRFLOW-3353

Description

Now that Celery/Kombu have updated and work with RedisPy 3.x (they in
fact force us to use 3.2) we should re-introduce this change.

This re-introduces the change from #4203 (with the addition of a note in UPDATING.md. The tests have been changed since so don't need updating anymore) that was reverted in #4205

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[ashb]

@Fokko @feng-tao PTAL - tests are failing until this is merged :(

[ashb]

Note to whoever merges this - please remove the (#4203) from the commit message (it snuck in when I took the author information from the previous PR.

[feng-tao]

lgtm, I am afk, and I think @ashb you could merge it on your own if you are by your computer.

[Fokko]

Thanks @ashb for picking this up so quickly

[ashb]

lgtm, I am afk, and I think @ashb you could merge it on your own if you are by your computer.

@feng-tao I generally try to get other committers to to merge my PRs still.

[feng-tao]

@ashb , I assume once other committers give +1, it should be fine. Besides we currently have a limited set of active committers.