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: I73e9613bf96da80d4dfd53246a568d0de691ae01

tox.ini

@@ -16,14 +16,17 @@ whitelist_externals =
   make
 
 [testenv:pep8]
+basepython = python3
 commands =
     python setup.py check --restructuredtext --strict
     flake8 openstackdocstheme
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands =
   rm -rf doc/build api-ref/build
   python setup.py build_sphinx
@@ -38,11 +41,13 @@ commands =
   sphinx-build -a -E -W -d api-ref/build/doctrees -b html api-ref/source api-ref/build/html
 
 [testenv:releasenotes]
+basepython = python3
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:api-ref]
+basepython = python3
 commands =
   rm -rf api-ref/build
   sphinx-build -a -E -W -d api-ref/build/doctrees -b html api-ref/source api-ref/build/html
@@ -66,6 +71,7 @@ commands =
 # system dependencies are missing, since it's used to tell you what system
 # dependencies are missing! This also means that bindep must be installed
 # separately, outside of the requirements files.
+basepython = python3
 deps = bindep
 commands = bindep test