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: Ic7138594becaf7061d8b4d0d3fe9bf243fef35af
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -67,9 +67,11 @@ commands =
     rm -rf .testrepository
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
     find . -type f -name "*.py[c|o]" -delete
     python setup.py test --coverage --coverage-package-name freezer_api --testr-args="{posargs}"
@@ -78,9 +80,11 @@ commands =
     rm -rf .testrepository
 
 [testenv:pylint]
+basepython = python3
 commands = pylint --rcfile .pylintrc freezer_api
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 freezer_api
 
 [flake8]
@@ -94,21 +98,25 @@ show-source = True
 exclude = .venv,.tox,dist,doc,*egg,specs,build
 
 [testenv:docs]
+basepython = python3
 commands =
   rm -rf api-ref/build
   python setup.py build_sphinx
   sphinx-build -W -b html api-ref/source api-ref/build/html
 
 [testenv:api-ref]
+basepython = python3
 whitelist_externals = rm
 commands =
   rm -rf api-ref/build
   sphinx-build -W -a -E -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -W -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:genpolicy]
+basepython = python3
 commands = oslopolicy-sample-generator --config-file etc/freezer/freezer-policy-generator.conf
 
 [testenv:lower-constraints]