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: Ifc0557996260608bbbb9437fe527e0fef5f05415

tox.ini

@@ -13,15 +13,18 @@ deps =
 commands = python manage.py test {posargs} --settings=congress_dashboard.test.settings
 
 [testenv:pep8]
+basepython = python3
 usedevelop = False
 deps =
 commands = {toxinidir}/tools/pip-install-single-req.sh test-requirements.txt hacking
            flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   coverage run {toxinidir}/manage.py test congress_dashboard --settings=congress_dashboard.test.settings {posargs} --exclude-dir=congress_dashboard/test/integration_tests {posargs}
@@ -29,12 +32,14 @@ commands =
   coverage html --omit '.tox/cover/*' -d 'cover/htmlcov'
 
 [testenv:docs]
+basepython = python3
 setenv = DJANGO_SETTINGS_MODULE=congress_dashboard.test.settings
 deps = -r{toxinidir}/doc/requirements.txt
 commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html