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: If74556d3489092ea33d82224d1c1695c182b6bea

tox.ini

@@ -26,6 +26,7 @@ setenv =
     OS_STDOUT_CAPTURE=1
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
@@ -33,6 +34,7 @@ basepython = python2
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
     rm -rf doc/html doc/build
@@ -40,6 +42,7 @@ commands =
     sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [flake8]
@@ -54,6 +57,7 @@ show-source = True
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build
 
 [testenv:bindep]
+basepython = python3
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed