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: Ib5b93ca7863fcc0b6d00fb4d52a5f299d1903056

.zuul.yaml

@@ -30,11 +30,12 @@
       tox_envlist: functional-py36
 
 - job:
-    name: python-cinderclient-functional-py38
+    name: python-cinderclient-functional-py39
     parent: python-cinderclient-functional-base
+    nodeset: devstack-single-node-centos-9-stream
     vars:
-      python_version: 3.8
-      tox_envlist: functional-py38
+      python_version: 3.9
+      tox_envlist: functional-py39
 
 - project:
     templates:
@@ -47,10 +48,10 @@
     check:
       jobs:
         - python-cinderclient-functional-py36
-        - python-cinderclient-functional-py38
+        - python-cinderclient-functional-py39
         - openstack-tox-pylint:
             voting: false
     gate:
       jobs:
         - python-cinderclient-functional-py36
-        - python-cinderclient-functional-py38
+        - python-cinderclient-functional-py39

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
 
 [files]
 packages =

tox.ini

@@ -100,12 +100,7 @@ setenv = {[testenv:functional]setenv}
 passenv = {[testenv:functional]passenv}
 commands = {[testenv:functional]commands}
 
-[testenv:functional-py37]
-setenv = {[testenv:functional]setenv}
-passenv = {[testenv:functional]passenv}
-commands = {[testenv:functional]commands}
-
-[testenv:functional-py38]
+[testenv:functional-py39]
 setenv = {[testenv:functional]setenv}
 passenv = {[testenv:functional]passenv}
 commands = {[testenv:functional]commands}