Drop python3.6/3.7 support in testing runtime

In Zed cycle testing runtime, we are targetting to drop the
python 3.6/3.7 support, project started adding python 3.8 as minimum,

example nova:
- 56b5aed08c/setup.cfg (L13)

Also moving zuul's job to `openstack-python3-jobs`.

Change-Id: I7997e1dc076ba4b9a385b96bfc97b7bf1c8aedd0

.zuul.yaml

@@ -1,6 +1,6 @@
 - project:
     templates:
-      - openstack-python3-yoga-jobs
+      - openstack-python3-jobs
       - build-openstack-docs-pti
     check:
       jobs:

setup.cfg

@@ -6,6 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://www.openstack.org/
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -14,5 +15,5 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9