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: I7e6d76b8dd2b037e766d11d1cf1560c0b08c637b

tox.ini

@@ -21,14 +21,17 @@ commands = {[unit_tests]commands}
 commands = python manage.py test {posargs} --settings=neutron_vpnaas_dashboard.test.settings
 
 [testenv:pep8]
+basepython = python3
 commands =
   flake8 {posargs}
   {envpython} {toxinidir}/manage.py extract_messages --module neutron_vpnaas_dashboard --verbosity 0 --check-only
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   coverage run {toxinidir}/manage.py test neutron_vpnaas_dashboard --settings=neutron_vpnaas_dashboard.test.settings {posargs}
@@ -42,12 +45,14 @@ commands =
   {[unit_tests]commands}
 
 [testenv:docs]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/doc/requirements.txt