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

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

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: I4444bfd4b729ccd86dabe1f2c37f475a3368a812

setup.cfg

@@ -6,6 +6,7 @@ description-file =
 author = OpenStack
 author-email = openstack-dev@lists.openstack.org
 home-page = http://www.openstack.org/
+python-requires = >=3.6
 classifier =
     Topic :: Utilities
     Intended Audience :: Developers
@@ -13,14 +14,10 @@ classifier =
     Intended Audience :: System Administrators
     License :: OSI Approved :: Apache Software License
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
-    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages =
     requestsexceptions
-
-[wheel]
-universal = 1

setup.py

@@ -16,14 +16,6 @@
 # THIS FILE IS MANAGED BY THE GLOBAL REQUIREMENTS REPO - DO NOT EDIT
 import setuptools
 
-# In python < 2.7.4, a lazy loading of package `pbr` will break
-# setuptools if some other modules registered functions in `atexit`.
-# solution from: http://bugs.python.org/issue15881#msg170215
-try:
-    import multiprocessing  # noqa
-except ImportError:
-    pass
-
 setuptools.setup(
     setup_requires=['pbr>=2.0.0'],
     pbr=True)

tox.ini

@@ -1,30 +1,27 @@
 [tox]
-minversion = 1.6
+minversion = 3.1.0
 skipsdist = True
-envlist = pep8, py27, py35
+envlist = pep8,py36
+ignore_basepython_conflict = true
 
 [testenv]
+basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:cover]
-basepython = python3
 commands =
   python setup.py testr --coverage
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [flake8]