Set the basepython for the venv tox environment

CI uses the venv tox environment to build the documentation. We need to
make sure that Python 2 is used until out docs build correctly under
Python 3.

Change-Id: I1a1d3123f22bf89965eb65e48648aae38cbdd254

tox.ini

@@ -47,6 +47,7 @@ commands =
 
 #set PYTHONHASHSEED=0 to prevent wsmeext.sphinxext from randomly failing.
 [testenv:venv]
+basepython = python2.7
 setenv = PYTHONHASHSEED=0
 commands = {posargs}