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: I3e2b7f9990b7d1b269a75070db6c4e6b056e4df7

tox.ini

@@ -18,12 +18,15 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/re
 commands = python manage.py test {posargs} --settings=blazar_dashboard.test.settings
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 {posargs}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   coverage run {toxinidir}/manage.py test blazar_dashboard --settings=blazar_dashboard.test.settings {posargs} --exclude-dir=blazar_dashboard/test/integration_tests {posargs}
@@ -37,6 +40,7 @@ commands =
   python manage.py test {posargs} --settings=blazar_dashboard.test.settings
 
 [testenv:eslint]
+basepython = python3
 whitelist_externals = npm
 commands =
   npm install
@@ -44,12 +48,14 @@ commands =
   npm run lint
 
 [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 = python setup.py build_sphinx
 
 [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