[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] pypa/setuptools#4483

Change-Id: I1b5f2e966290c4f5aa87f800f9cb4ae6e84a3889

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - check-requirements
       - lib-forward-testing-python3
-      - openstack-lower-constraints-jobs
       - openstack-python3-victoria-jobs
       - periodic-stable-jobs
       - publish-openstack-docs-pti

tox.ini

@@ -2,6 +2,12 @@
 minversion = 3.2.0
 envlist = py37,pep8
 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