Add Python 3 only classifier

Python 2 support was removed during Ussuri cycle. This change adds
the classifier to clearly state that only Python 3 is supported.

Change-Id: Id801919bbe0f3fcc23006911512bdf6fafa3d395

setup.cfg

@@ -6,6 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page = http://www.openstack.org/
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -13,6 +14,7 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
+    Programming Language :: Python :: 3 :: Only
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7