Switch to Python3 zed unit test jobs

Ensure unit testing is in place for all of the tested
runtimes for zed.

Change-Id: If17bc27f244aea3a0989873b99024f15e224e7d3

.zuul.yaml

@@ -1,4 +1,4 @@
 - project:
     templates:
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/renderspec/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Environment :: Console
     Environment :: OpenStack
@@ -18,9 +18,8 @@ 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
 
 [entry_points]
 console_scripts =