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 do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: Idac327ef35d8317c6e9ae838f2d88c4b5b926606

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py27,py35,pep8,py27dj19,py27dj110,py3-dj111
+envlist = py35,py27,pep8,py27dj19,py27dj110,py3-dj111
 skipsdist = True
 
 [testenv]
@@ -19,6 +19,7 @@ setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
 setenv = DJANGO_SETTINGS_MODULE=manila_ui.tests.settings
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
@@ -47,19 +48,23 @@ commands =
   {[testenv]commands}
 
 [testenv:cover]
+basepython = python3
 commands = {toxinidir}/tools/cover.sh {posargs}
 
 [testenv:docs]
+basepython = python3
 whitelist_externals = rm
 commands =
   rm -rf doc/build
   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]
@@ -71,9 +76,7 @@ enable-extensions = H203,H106
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,.ropeproject,tools
 
 [testenv:lower-constraints]
-# TODO: basepython must be switched to python3
-# after manila-ui supports python3.
-# basepython = python3
+basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt