Update supported python versions

Python 3.9, 3.10 and 3.11 are now part of tested runtimes and we
actually test these versions.

Also Python 3.6 test is added so that the minimum version is actually
tested.

Change-Id: I1cb6a0dc6c2cd18eaf64ead4d4e8b3d686f1ecee

.zuul.yaml

@@ -3,3 +3,9 @@
       - openstack-python3-jobs
       - openstack-cover-jobs
       - publish-openstack-docs-pti
+    check:
+      jobs:
+        - openstack-tox-py36
+    gate:
+      jobs:
+        - openstack-tox-py36

setup.cfg

@@ -18,6 +18,9 @@ classifier =
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
+    Programming Language :: Python :: 3.11
     Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: Implementation :: CPython