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

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

Tempest is ready with python 3 and ok to drop the
python 2.7 support. Last Tempest version officially supported
for py2.7 is 23.0.0.

Now onwards, Tempest will test all stable branch with py3 jobs.
tempest-full and other py2 version of jobs definition are kept
for stable branch 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: I75868d5c9b6630fe78958ff89e58a0aced09a6b3
diff --git a/setup.cfg b/setup.cfg
index dd30069..52c31db 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -6,6 +6,7 @@
 author = OpenStack
 author-email = openstack-discuss@lists.openstack.org
 home-page = https://docs.openstack.org/tempest/latest/
+requires-python = >=3.6
 classifier =
     Intended Audience :: Information Technology
     Intended Audience :: System Administrators
@@ -13,11 +14,11 @@
     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 :: 3.6
     Programming Language :: Python :: 3.7
+    Programming Language :: Python :: 3 :: Only
+    Programming Language :: Python :: Implementation :: CPython
 
 [files]
 packages =