Update CI to use unversioned jobs templat

As part of the migration of this project to the independent release
model, we failed to notice that the job template was still tied to a
specific release. We've now introduced a new unversioned job template,
'openstack-python3-jobs' [1], which was can and should use. Do this.

[1] https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/

Change-Id: I7d39046a59f08d60fcff1f4b28287c6979a803ed

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - docs-on-readthedocs
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-jobs
     vars:
       rtd_webhook_id: '47123'
 

setup.cfg

@@ -7,6 +7,7 @@ description-content-type = text/x-rst
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://www.openstack.org/
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Developers
@@ -14,10 +15,11 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
+    Programming Language :: Python :: Implementation :: CPython
-    Programming Language :: Python :: 2.7
+    Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py35,py27,pep8,docs,py39
+envlist = py3,pep8,docs
 minversion = 2.0
 skipsdist = True