Remove support and automated checks for Python 2.x

As Python 2 has reached EOL, is time to remove the declarative support
for it. This code snapshot still supports Python 2.7, but in the future,
no Python 2.x tests will be performed to ensure the support.

Change-Id: Ic8fe6b958d658ae25f887179ab5900e13c88af54

Author: ader1990

.zuul.yaml

@@ -2,7 +2,6 @@
     templates:
       - openstack-cover-jobs
       - build-openstack-docs-pti
-      - openstack-python-jobs
       - openstack-python3-ussuri-jobs
       - build-release-notes-jobs-python3
     post:

setup.cfg

@@ -14,8 +14,6 @@ classifier =
   License :: OSI Approved :: Apache Software License
   Operating System :: OS Independent
   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