[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: I4b491c076f7ba28cdebd77341bd20bcf64e58e66

tox.ini

@@ -1,9 +1,11 @@
 [tox]
-minversion = 1.6
-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}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
@@ -13,17 +15,14 @@ commands = python setup.py testr --slowest --testr-args='{posargs}'
 whitelist_externals = find
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands =
     find . -type f -name "*.pyc" -delete
     python setup.py build_sphinx
 
 [testenv:doc8]
-basepython = python3
 deps =
     -r{toxinidir}/requirements.txt
     doc8