[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: I294f6715c41fd1651e92f41c6601880c19f442a7

tox.ini

@@ -6,6 +6,11 @@ skipsdist = True
 # env and ignore basepython inherited from [testenv] if we set
 # ignore_basepython_conflict.
 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 = python3