[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

heat-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

Stable zuul jobs are modified to run with py3 as heat-tempest-plugin
cannot run py2 jobs on master gate.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Ida6919a70dc8711166a79eac574b0714382e8d00
diff --git a/setup.cfg b/setup.cfg
index 3bdd080..8776610 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -13,10 +13,10 @@
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 keywords =
     setup