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: Ia481330b9a889b113b585fca0d4ddb86df9f74d3
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -18,35 +18,42 @@ commands =
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
+basepython = python3
 commands =
     doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
     flake8
     bandit -r watcher -x tests -n5 -ll -s B320
 
 [testenv:venv]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
     python setup.py testr --coverage --testr-args='{posargs}'
     coverage report
 
 [testenv:docs]
+basepython = python3
 setenv = PYTHONHASHSEED=0
 commands =
     doc8 doc/source/ CONTRIBUTING.rst HACKING.rst README.rst
     python setup.py build_sphinx
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t watcher/tests {posargs}
 
 [testenv:genconfig]
+basepython = python3
 sitepackages = False
 commands =
     oslo-config-generator --config-file etc/watcher/oslo-config-generator/watcher.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands =
     oslopolicy-sample-generator --config-file etc/watcher/oslo-policy-generator/watcher-policy-generator.conf
 
@@ -59,6 +66,7 @@ enable-extensions = H106,H203,H904
 exclude=.venv,.git,.tox,dist,doc,*lib/python*,*egg,build,*sqlalchemy/alembic/versions/*,demo/,releasenotes
 
 [testenv:wheel]
+basepython = python3
 commands = python setup.py bdist_wheel
 
 [hacking]
@@ -71,9 +79,11 @@ extension=.rst
 ignore-path=doc/source/image_src,doc/source/man,doc/source/api
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:bandit]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = bandit -r watcher -x tests -n5 -ll -s B320