Update to zed cycle testing runtime

Fix Zed gate for python-watcherclient by updating to Zed testing
runtime.

Change-Id: I49f1156065a9a405691383af755634be7d29af31

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-python3-yoga-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - openstackclient-plugin-jobs

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page =  https://docs.openstack.org/python-watcherclient/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,9 +15,8 @@ classifier =
     Operating System :: POSIX :: Linux
     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
 
 [files]
 packages =