[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] https://github.com/pypa/setuptools/issues/4483

Closes-Bug: #2082234
Change-Id: I08e57e3b4c905bd679038c89185e54f6ab79738a

tox.ini

@@ -3,6 +3,11 @@ envlist = docs,py38,pep8
 minversion = 3.18.0
 skipsdist = False
 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]
 basepython = {env:TOX_PYTHON:python3}