Updating python testing as per Yoga testing runtime

Yoga testing runtime[1] has been updated to add py39
testing as voting. Unit tests update are
handled by the job template change in openstack-zuul-job

- https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/820286

this commit makes other required changes in zuul.yaml and
update the classifier in setup.cfg file.

[1] https://governance.openstack.org/tc/reference/runtimes/yoga.html

Change-Id: Ia7cd1aaf66714d7d885187fa14627d4d5c424f65

.zuul.yaml

@@ -13,7 +13,7 @@
 - project:
     templates:
       - check-requirements
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-yoga-jobs
       - openstackclient-plugin-jobs
       - publish-openstack-docs-pti
     check:

setup.cfg

@@ -17,7 +17,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
-
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py38,py36,pep8
+envlist = py38,py39,pep8
 skipsdist = True
 ignore_basepython_conflict = True