[unmaintained-only] Cap setuptools via virtualenv<20.26.4

py39 jobs (on ubuntu-focal) started to fail due to recent virtualenv
release (20.26.4 that bundles setuptools 74.1.2) on Yoga, because
we have 'packaging==21.3' in this branch that is not compatible
with the new 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: I668557933c51123ce84275a0c718b6e79b3df174

Author: elajkat

tox.ini

@@ -6,6 +6,12 @@ skip_missing_interpreters = true
 # 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]
 # Set default python version