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: If51ceaf9c96a886b17d3bc793176c839e06d0b72
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -17,14 +17,17 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands =
   find . -type f -name "*.pyc" -delete
   python setup.py build_sphinx
 
 [testenv:pep8]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands =
   flake8 {posargs}