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

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

zaqar-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://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I2417f29592bb87c9e3071b1918515a05f77cf8a3

releasenotes/notes/drop-py-2-7-8a4da4bd0e7c36f5.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of zaqar-tempest-plugin
+    to support py2.7 is OpenStack Train. The minimum version of Python now
+    supported by zaqar-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 :: 3.3
     Programming Language :: Python :: 3.4

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 2.0
-envlist = py35,py27,pypy,pep8
+minversion = 3.1.1
+envlist = py35,pypy,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv =