Updating python testing as per Yoga testing runtime

Yoga testing runtime[1] has been updated to add py39
testing as voting and as we are testing py3.6 and py3.9
we do not need to test py3.7|8 explicitly. 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: I9fe6187ea8c849812a508b48b342226e50fca831

.zuul.yaml

@@ -24,7 +24,7 @@
               - ^contrib/block-box.*$
               - ^doc/.*$
               - ^releasenotes/.*$
-        - openstack-tox-functional-py38:
+        - openstack-tox-functional-py39:
             irrelevant-files: *functional-irrelevant-files
         - cinder-rally-task:
             voting: false

setup.cfg

@@ -20,6 +20,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 project_urls:
     Source=https://opendev.org/openstack/cinder
     Tracker=https://bugs.launchpad.net/cinder

tox.ini

@@ -58,6 +58,10 @@ setenv =
 setenv =
   {[testenv:functional]setenv}
 
+[testenv:functional-py39]
+setenv =
+  {[testenv:functional]setenv}
+
 [testenv:api-samples]
 envdir = {toxworkdir}/shared
 setenv =