fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We can't set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36 due to https://github.com/tox-dev/tox/issues/425

Change-Id: I9abfcb14c06a6a9d31e9f53278dd11eccdef488d

tox.ini

@@ -4,7 +4,6 @@ envlist = py34,py27,pypy,pep8
 skipsdist = True
 
 [testenv]
-basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 setenv =
@@ -15,26 +14,29 @@ deps =
   -r{toxinidir}/test-requirements.txt
 commands = python setup.py test --slowest --testr-args='{posargs}'
 
-[testenv:py27]
-basepython = python2.7
-
 [testenv:pep8]
+basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py test --coverage --testr-args='{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
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [flake8]