[stable-only] Cap setuptools <71.0.0

py38 jobs (on ubuntu-focal) started to fail (see the relevant github
issue [1]) due to recent virtualenv release (20.26.4; which bundles
setuptools).

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.

Furthermore, lower-constraints jobs are not supported anymore and are
removed from most of the branches, so let's remove here as well to
unblock the gate.

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

Change-Id: I883459af0e8bae3b7e2f70ead65ae1e05b2621ba

.zuul.yaml

@@ -81,7 +81,6 @@
       - check-requirements
       - lib-forward-testing-python3
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python3-victoria-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

tox.ini

@@ -2,6 +2,12 @@
 envlist = py38,pep8
 minversion = 2.0
 skipsdist = 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]
 usedevelop = True