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: Ife18281cb3f1a28d1ccc52196ca708e7ace14395
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -13,9 +13,11 @@ deps = -r{toxinidir}/requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:spelling]
@@ -26,6 +28,7 @@ deps =
 commands = sphinx-build -b spelling doc/source doc/build/spelling
 
 [testenv:cover]
+basepython = python3
 setenv =
     PYTHON=coverage run --source specs --parallel-mode
 commands =
@@ -35,6 +38,7 @@ commands =
     coverage xml -o cover/coverage.xml
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [flake8]