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: I6e78e80c2e5c4e5a39bd3c3dfbfddaf3973abb77

.zuul.yaml

@@ -2,4 +2,4 @@
     templates:
       - horizon-non-primary-django-jobs
       - check-requirements
-      - openstack-python3-xena-jobs
+      - openstack-python3-zed-jobs

releasenotes/notes/drop-python-3-6-and-3-7-ae37bc21f97de767.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

@@ -3,12 +3,12 @@ name = mistral-dashboard
 summary = Mistral dashboard
 description_file = README.rst
 license = Apache License, Version 2.0
+python_requires = >=3.8
 classifiers =
     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
     Environment :: OpenStack
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators