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: Ib4f4e6ceba58a94e512ca1f3329b2a4a6c5d02bd

.zuul.yaml

@@ -54,11 +54,12 @@
       tox_envlist: functional-py36
 
 - job:
-    name: brick-cinderclient-dsvm-functional-py38
+    name: brick-cinderclient-dsvm-functional-py39
     parent: brick-cinderclient-dsvm-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:
@@ -69,8 +70,8 @@
     check:
       jobs:
         - brick-cinderclient-dsvm-functional-py36
-        - brick-cinderclient-dsvm-functional-py38
+        - brick-cinderclient-dsvm-functional-py39
     gate:
       jobs:
         - brick-cinderclient-dsvm-functional-py36
-        - brick-cinderclient-dsvm-functional-py38
+        - brick-cinderclient-dsvm-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

@@ -47,7 +47,7 @@ passenv = OS_*
 setenv =
   {[testenv:functional]setenv}
 
-[testenv:functional-py38]
+[testenv:functional-py39]
 setenv =
   {[testenv:functional]setenv}