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: I03d26060e9b36ed50062a54b9de29d7dc6787fb9
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -18,29 +18,36 @@ commands =
   python setup.py testr --testr-args='{posargs}'
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8 {posargs} cloudkitty
 
 [testenv:cover]
+basepython = python3
 setenv = VIRTUAL_ENV={envdir}
 commands =
   python setup.py testr --coverage --testr-args='{posargs}'
   coverage report
 
 [testenv:genconfig]
+basepython = python3
 commands =
   oslo-config-generator --config-file etc/oslo-config-generator/cloudkitty.conf
 
 [testenv:genpolicy]
+basepython = python3
 commands = oslopolicy-sample-generator --config-file=etc/oslo-policy-generator/cloudkitty.conf
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [flake8]
@@ -53,4 +60,5 @@ import_exceptions = cloudkitty.i18n
 local-check-factory = cloudkitty.hacking.checks.factory
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html