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: Iebd573f07fde5cae14b48a89c7292a923ff03aee

.zuul.yaml

@@ -3,7 +3,7 @@
       - check-requirements
       - horizon-non-primary-django-jobs
       - openstack-lower-constraints-jobs-horizon
-      - openstack-python3-yoga-jobs-horizon
+      - openstack-python3-zed-jobs-horizon
       - release-notes-jobs-python3
     experimental:
       jobs:

releasenotes/notes/drop-python-3-6-and-3-7-a6d76484c658a01b.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/sahara/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,8 +17,6 @@ classifier =
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9