Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Add release notes and update the python
classifier for the same.

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

Co-Authored-By: Ghanshyam Mann <gmann@ghanshyammann.com>
Change-Id: I266bfd155742abcbdafa0813eb955b864fcf3adc

.zuul.yaml

@@ -3,6 +3,6 @@
       - check-requirements
       - horizon-non-primary-django-jobs
       - openstack-lower-constraints-jobs-horizon
-      - openstack-python3-wallaby-jobs-horizon
+      - openstack-python3-zed-jobs-horizon
       - publish-openstack-docs-pti
       - release-notes-jobs-python3

releasenotes/notes/drop-python-3-6-and-3-7-39c229fcb744870f.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/neutron-vpnaas-dashboard/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,9 +15,8 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =