Update python testing as per zed cycle teting runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Add release notes, move py36 jobs to py38|9,
and update the python classifier for the same.

[1] https://governance.openstack.org/tc/reference/runtimes/zed.html

Change-Id: Ibeee0b16ee1ea95f05127563a9cb08ffd0f04e2b

.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
       - lib-forward-testing-python3

releasenotes/notes/drop-python-3-6-and-3-7-73767fa0bbe89a6e.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Python 3.6 & 3.7 support has been dropped. The minimum version of Python now
+    supported is Python 3.8.

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack Networking Project
 author_email = openstack-discuss@lists.openstack.org
 home_page = https://docs.openstack.org/python-neutronclient/latest/
-python_requires = >=3.6
+python_requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Developers
@@ -18,8 +18,6 @@ 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