[ussuri][goal] Drop python2.7 support

As per the official python support timeline
set forth by the OpenStack TC [1][2], OpenStack
Train is the last release that will support
python2.7. manila-tempest-plugin is a 'branchless'
tempest plugin. However, after this change,
users may no longer expect to install this package
with python2.7.

Depends-On: https://review.opendev.org/#/c/688997/
[1] https://governance.openstack.org/tc/resolutions/20180529-python2-deprecation-timeline.html
[2] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: I8c10d169302c6f1c41e238ae9f787455a2b15a4d
diff --git a/tox.ini b/tox.ini
index 00b23d0..0218477 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,9 +1,10 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,pypy,pep8
+envlist = py3,pypy,pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
@@ -17,19 +18,15 @@
 # commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
-basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:docs]
-basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:releasenotes]
-basepython = python3
 commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html