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: Ief7a2b6846c3035d4d0a17ad54768d483da14802
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -29,6 +29,7 @@ commands =
   coverage xml -o cover/coverage.xml
 
 [testenv:venv]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
@@ -40,11 +41,13 @@ passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY \
           OS_USER_DOMAIN_NAME OS_PROJECT_DOMAIN_NAME OS_CLOUD OS_CACERT
 
 [testenv:pep8]
+basepython = python3
 commands =
     flake8 metalsmith
     doc8 README.rst doc/source roles
 
 [testenv:docs]
+basepython = python3
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/doc/joined-requirements.txt