Update python testing as per zed cycle teting 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: Ic7671ad666ebc0614686e068b9d4c0824e384fb3

.zuul.yaml

@@ -2,7 +2,7 @@
     templates:
       - check-requirements
       - lib-forward-testing-python3
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-zed-jobs
       - openstacksdk-functional-tips
       - openstacksdk-tox-tips
       - osc-tox-unit-tips

releasenotes/notes/drop-python-3-6-and-3-7-c407d5898c5eafec.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/keystoneauth/latest/
-python_equires = >=3.6
+python_equires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,9 +17,8 @@ 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
 
 [files]
 packages =

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.1.1
 skipsdist = True
-envlist = py38,pep8,releasenotes
+envlist = py39,pep8,releasenotes
 ignore_basepython_conflict = True
 
 [testenv]