[stable-only] Cap setuptools <71.0.0

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.

[1] pypa/setuptools#4483

Change-Id: Ibbfef10e65d4acea6dd4e9f8a4fb3a4bcc877549

tox.ini

@@ -1,14 +1,17 @@
 [tox]
 minversion = 3.18.0
-requires =
-    virtualenv>=20.4.2
-    tox<4
 skipsdist = True
 # python runtimes: https://governance.openstack.org/tc/reference/project-testing-interface.html#tested-runtimes
 envlist = py3,compliance,pep8
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file
 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
+  setuptools<71.0.0
 
 [testenv]
 basepython = python3