[ussuri][goal] Drop python 2.7 support

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

specs repo either has py27 job or requirement or tox env.

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Icbfbc6503a21c5a8d37d527eaf5f217c2beab5da

tox.ini

@@ -1,11 +1,11 @@
 [tox]
-# Hold back to 1.4, since that's what's in Fedora 20 repos
-# and we don't need anything newer for nova-specs tests
-minversion = 1.4
-envlist = docs,py27
+minversion = 3.1.1
+envlist = docs,pep8
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 setenv = VIRTUAL_ENV={envdir}
 install_command = pip install -U {opts} {packages}
@@ -13,9 +13,7 @@ deps = -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx