[stable-only] Cap setuptools <71.0.0 and fix gate

py39 jobs (on ubuntu-focal) started to fail due to recent virtualenv
release (20.26.4) on Yoga (which bundles setuptools), because we have
'packaging==21.3' in this branch that is not compatible with newer
setuptools [1].

setuptools is bundled in virtualenv, so it has to be capped via the
virtualenv package. tox also needed to be capped (<4) as gate uses
tox 3.28.0, but with capping virtualenv we pull in latest tox as well,
which would cause other errors.

This patch also sets to use keystone-tempest-plugin from wallaby-last
tag for functional jobs otherwise they fail [2] as on newer versions
of keystone-tempest-plugin keystoneauth is set as >=5.1.1 in
requirements.txt however we pin keystoneauth in upper-constraints.txt
much lower version than that.

[1] pypa/setuptools#4483
[2] ERROR: Cannot install keystone-tempest-plugin==0.17.0 because these
package versions have conflicting dependencies.
The conflict is caused by:
    keystone-tempest-plugin 0.17.0 depends on keystoneauth1>=5.1.1
    The user requested (constraint) keystoneauth1===4.5.0

Change-Id: I76c39af6119806631d800ad644db270dd7d01b64

Author: Elod Illes

.zuul.yaml

@@ -16,7 +16,8 @@
     timeout: 4200
     required-projects:
       - openstack/keystone
-      - openstack/keystone-tempest-plugin
+      - name: openstack/keystone-tempest-plugin
+        override-checkout: wallaby-last
     vars:
       tox_envlist: all
       tempest_test_regex: 'keystone_tempest_plugin'

tox.ini

@@ -2,6 +2,11 @@
 minversion = 3.2.0
 envlist = py37,pep8,api-ref,docs,genconfig,genpolicy,releasenotes,protection
 ignore_basepython_conflict = true
+# Cap setuptools via virtualenv to prevent compatibility issue with yoga
+# branch's upper constraint of 'packaging' package (21.3).
+requires =
+  virtualenv<20.26.4
+  tox<4
 
 [testenv]
 usedevelop = True