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: I0a40f91edc9a7789842384f6f4ced0f229c183ef

tox.ini

@@ -21,6 +21,7 @@ deps = -r{toxinidir}/requirements.txt
 commands = {toxinidir}/manage.py test muranodashboard --settings=muranodashboard.tests.settings
 
 [testenv:pep8]
+basepython = python3
 sitepackages = False
 commands = flake8
 
@@ -31,9 +32,11 @@ deps = -r{toxinidir}/requirements.txt
        http://tarballs.openstack.org/horizon/horizon-stable-ocata.tar.gz
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = {toxinidir}/tools/cover.sh {posargs}
 
 [testenv:pyflakes]
@@ -48,9 +51,11 @@ commands = nodeenv -p
            npm run lint
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:makemessages]