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: If27b1960674af44a4de7af412550321d74f05af6
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -14,15 +14,18 @@ commands = rm -f .testrepository/times.dbm
            python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:common-constraints]
+basepython = python3
 install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 
 [testenv:genpolicy]
+basepython = python3
 sitepackages = False
 envdir = {toxworkdir}/venv
 commands =
     oslopolicy-sample-generator --config-file=tools/config/cyborg-policy-generator.conf
 
 [testenv:genconfig]
+basepython = python3
 sitepackages = False
 envdir = {toxworkdir}/venv
 commands =
@@ -30,17 +33,21 @@ commands =
 
 
 [testenv:pep8]
+basepython = python3
 commands = pep8 {posargs}
            doc8 {posargs}
 
 [testenv:pep8-constraints]
+basepython = python3
 install_command = {[testenv:common-constraints]install_command}
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
 [doc8]
@@ -48,14 +55,17 @@ ignore-path = .venv,.git,.tox,*cyborg/locale*,*lib/python*,*cyborg.egg*,api-ref/
 
 
 [testenv:docs]
+basepython = python3
 commands =
     python setup.py build_sphinx
     oslo-config-generator --config-file=tools/config/cyborg-config-generator.conf
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t cyborg/tests {posargs}
 
 [pep8]