Update CI to use unversioned jobs template

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/

Depends-On: https://review.opendev.org/c/openstack/openstack-zuul-jobs/+/833286/

Change-Id: I6475b3a47b4f344e78ddfdd1d1d093e7eef0bb81

.zuul.yaml

@@ -1,19 +1,6 @@
 - project:
     templates:
       - check-requirements
+      - openstack-python3-jobs
       - publish-openstack-docs-pti
       - release-notes-jobs-python3
-    check:
-      jobs:
-        - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
-        - openstack-tox-py38
-        - openstack-tox-py39
-    gate:
-      jobs:
-        - openstack-tox-pep8
-        - openstack-tox-py36
-        - openstack-tox-py37
-        - openstack-tox-py38
-        - openstack-tox-py39

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page =  https://docs.openstack.org/os-testr/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,8 +15,6 @@ 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
     Programming Language :: Python :: 3 :: Only