Continue using tox 3

Use the classic cinderclient tox.ini that has worked so well in the
past with tox 3; only change is to add the tox<4 requires statement.
Also adjust .zuul.yaml to express our preference for tox<4 in the
case that zuul has to install tox.

Change-Id: Ib1f55f9431033ad043507c6f751ee9bfe57d5cbb

.zuul.yaml

@@ -38,6 +38,8 @@
       tox_envlist: functional-py39
 
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - check-requirements
       - lib-forward-testing-python3

tox.ini

@@ -2,6 +2,7 @@
 distribute = False
 envlist = py3,pep8
 minversion = 3.18.0
+requires = tox<4
 skipsdist = True
 # this allows tox to infer the base python from the environment name
 # and override any basepython configured in this file