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.d/cross-jobs.yaml``,
tox.ini and update the classifier in setup.cfg file.

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

Change-Id: Ie66460c19e5e670e9be3b1262d5f06e821078be9

.zuul.d/cross-jobs.yaml

@@ -11,7 +11,7 @@
 
 - job:
     name: horizon-cross-manila-ui-python
-    parent: openstack-tox-py38
+    parent: openstack-tox-py39
     vars:
       zuul_work_dir: "{{ zuul.projects['opendev.org/openstack/manila-ui'].src_dir }}"
     required-projects:
@@ -28,7 +28,7 @@
 
 - job:
     name: horizon-cross-heat-dashboard-python
-    parent: openstack-tox-py38
+    parent: openstack-tox-py39
     vars:
       zuul_work_dir: "{{ zuul.projects['opendev.org/openstack/heat-dashboard'].src_dir }}"
     required-projects:

setup.cfg

@@ -21,6 +21,7 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
     Topic :: Internet :: WWW/HTTP
 
 [global]

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1
-envlist = pep8,py36,py38,releasenotes,npm,py3-dj32
+envlist = pep8,py36,py39,releasenotes,npm,py3-dj32
 skipsdist = True
 # Automatic envs (pyXX) will only use the python version appropriate to that
 # env and ignore basepython inherited from [testenv] if we set