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. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.

Change-Id: I09f5c8abfbf33a628af4f2157164c588e725eac6

.zuul.yaml

@@ -1,6 +1,6 @@
 - project:
     templates:
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-zed-jobs
       - check-requirements
     gate:
       queue: os-win

releasenotes/notes/drop-python-3-6-and-3-7-fbf1c519725da78b.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

@@ -6,6 +6,7 @@ description-file =
 author = Cloudbase Solutions
 author-email = info@cloudbasesolutions.com
 home-page = https://opendev.org/openstack/oswin-tempest-plugin
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,8 +16,6 @@ classifier =
     Operating System :: Microsoft :: Windows
     Programming Language :: Python
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
 
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