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.

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

Change-Id: I817a4d1506fb7f15e72d37015ae0ba9547e2aa52

.zuul.yaml

@@ -203,7 +203,7 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-python3-xena-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti
       - periodic-stable-jobs
       - check-requirements

releasenotes/notes/drop-python-3-6-and-3-7-dc90b86cedced92b.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/keystone/latest
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -17,8 +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 :: 3.8
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.9
 
 [files]
 data_files =

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.2.0
-envlist = py37,pep8,api-ref,docs,genconfig,genpolicy,releasenotes,protection
+envlist = py39,pep8,api-ref,docs,genconfig,genpolicy,releasenotes,protection
 ignore_basepython_conflict = true
 
 [testenv]