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: I78cd11a853bff71e22727561a79b17d3520bd5d6

tox.ini

@@ -36,12 +36,15 @@ basepython = python3.5
 commands = oslo_debug_helper -t kuryr_libnetwork/tests {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 setenv =
     VIRTUAL_ENV={envdir}
     PYTHON=coverage run --source kuryr_libnetwork --parallel-mode
@@ -53,6 +56,7 @@ commands =
     coverage report
 
 [testenv:docs]
+basepython = python3
 commands = python setup.py build_sphinx
 
 [flake8]
@@ -63,6 +67,7 @@ show-source = true
 exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,tools,.ropeproject,rally-scenarios,releasenotes
 
 [testenv:pylint]
+basepython = python3
 deps =
   {[testenv]deps}
   pylint
@@ -74,9 +79,11 @@ import_exceptions = kuryr.lib._i18n
 local-check-factory = neutron_lib.hacking.checks.factory
 
 [testenv:genconfig]
+basepython = python3
 commands = {toxinidir}/tools/generate_config_file_samples.sh
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]