Drop python 2.7 support and testing

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

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

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://review.opendev.org/#/c/691178/

Change-Id: I4ef56685bb9482cb9a27087841c537f5805bd4e2

releasenotes/notes/drop-py-2-7-dade142c6d1e6a21.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of solum-tempest-plugin
+    to support python 2.7 is OpenStack Train. The minimum version of Python now
+    supported by solum-tempest-plugin is Python 3.6.

setup.cfg

@@ -13,8 +13,6 @@ classifier =
     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

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py37,py36,py27,pypy,pep8
+envlist = py37,py36,pypy,pep8
 skipsdist = True
 
 [testenv]