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: If11afb223605f476996f5147a7e10dc0dfb8f00b

tox.ini

@@ -17,9 +17,11 @@ commands =
     stestr run --slowest {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 commands =
     find . -type f -name "*.pyc" -delete
@@ -27,6 +29,7 @@ commands =
     python setup.py build_sphinx
 
 [testenv:pep8]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands =
     doc8 doc/source/ README.rst