Update zuul jobs for Xena cycle

Use Xena zuul job template which contains also py39 job
and add jobs to run refstack-client also on stable/wallaby
release.
Also per Xena testing runtime we need to support at least
py36 and py38:
https://governance.openstack.org/tc/reference/runtimes/xena.html

Depends-On: https://review.opendev.org/c/openstack/devstack/+/790353
Change-Id: Id65875ab48ce1b25bd929cb784a86a093c586e73

.zuul.yaml

@@ -2,16 +2,18 @@
     templates:
       - openstack-cover-jobs
       - openstack-python37-jobs
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-xena-jobs
     check:
       jobs:
         - refstack-client-devstack-master
+        - refstack-client-devstack-wallaby
         - refstack-client-devstack-victoria
         - refstack-client-devstack-ussuri
         - refstack-client-devstack-train
     gate:
       jobs:
         - refstack-client-devstack-master
+        - refstack-client-devstack-wallaby
         - refstack-client-devstack-victoria
         - refstack-client-devstack-ussuri
         - refstack-client-devstack-train

setup.cfg

@@ -20,6 +20,7 @@ classifier =
     Programming Language :: Python :: 3.5
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py35,py36,py37,py27
+envlist = pep8,py35,py36,py38,py27
 minversion = 3.18
 skipsdist = True