Update python testing as per zed cycle testing runtime

In Zed cycle, we have dropped the python 3.6/3.7[1] testing
and its support. Removing the py36 centos8 job as well as
updating the python classifier also to reflect the same.
Also indicates that we support python 3.9.

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

Change-Id: I36180f40cb2858aa051366af9b3c3c28d635632c

.zuul.yaml

@@ -2,7 +2,7 @@
     templates:
       - check-requirements
       - openstack-cover-jobs
-      - openstack-python3-wallaby-jobs
+      - openstack-python3-zed-jobs
       - publish-openstack-docs-pti
       - build-release-notes-jobs-python3
     check:

setup.cfg

@@ -6,7 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/python-magnumclient/latest/
-python-requires = >=3.6
+python-requires = >=3.8
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -16,11 +16,9 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 2
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.6
-    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
 
 [files]
 packages =