Continue to use tox 3 in the stable branches.

Two changes:
- .zuul.yaml: set the ensure_tox_version to use <4, which will
  tell zuul to install tox<4 if it's not present
- tox.ini: set requires=tox<4 so that if tox has been installed
  already, our tox-based jobs will use tox 3 to run the tests

Change-Id: Ica4c0a1d4f861e528ce8995766e82541dc710e0f
(cherry picked from commit 22a3169576b3fd91388e7b57ffd39e3760016796)
(cherry picked from commit e3799da5558a6a686885fa70c155d8d37d44497d)

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/xena branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I6b7c345706fbc0c257296273a5eaa406ccdf47e5

Change-Id: Id71184f99c448b71bb6d437c14d293a10a15f154

Includes prelude plus a note for change I524490aa988f.

Change-Id: I233faad57c9708cae9544c965fd0d94abdf6d684

Block Storage API mv 3.66 enables snapshots of in-use volumes
without requiring a 'force' flag.  For backward compatibility,
the API silently accepts force=true, even though the 'force' flag
is considered invalid for that call.  That behavior is replicated
in the client, where --force with a true value is silently accepted.
The --force option is not advertised in the shell and an option
value that doesn't evaluate to true raises an UnsupportedAttribute
error.  Similar behavior from the v3 Snapshot class, except it
raises a ValueError under similar circumstances.

Change-Id: I7408d0e3a5ed7f4cbcaf65cf3434ad60aaed511d

remove the the colorizer.py which hasn't been used 
since https://review.opendev.org/c/openstack/python-cinderclient/+/502120

Change-Id: I9bed2ec947705cd53daa04678093a80581dc3282