Test python3.9 as voting job

In Yoga testing, we are making py3.9 job to run as voting, so
adding py3.9 job as voting.

Also, as Tempest is branchless and needs to support older
supported stable branches, let's test it with all the
python version supported in those supported stable branches.
For that, we need to explicitly dd the py jobs instead of using
the template. This way, we will explicitly know that we are testing
all supported python versions when the new release template bumps the
min python version in the template.

Change-Id: I71b2dbfa46449e39b6fbe99862c55e4370ce68c3

.zuul.yaml

@@ -2,6 +2,20 @@
     templates:
       - check-requirements
       - publish-openstack-docs-pti
-      - openstack-python35-jobs
-      - openstack-python3-yoga-jobs
       - openstack-cover-jobs
+    check:
+      jobs:
+        - openstack-tox-pep8
+        - openstack-tox-py35
+        - openstack-tox-py36
+        - openstack-tox-py37
+        - openstack-tox-py38
+        - openstack-tox-py39
+    gate:
+      jobs:
+        - openstack-tox-pep8
+        - openstack-tox-py35   
+        - openstack-tox-py36
+        - openstack-tox-py37
+        - openstack-tox-py38
+        - openstack-tox-py39

setup.cfg

@@ -19,6 +19,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython