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: I634e5ed6236ff0253b713b411ab203072485af2c

tox.ini

@@ -33,12 +33,15 @@ basepython = python3.5
 commands = oslo_debug_helper {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   rm -f .testrepository/times.dbm
   python setup.py test --coverage --testr-args='{posargs}' \
@@ -46,6 +49,7 @@ commands =
   coverage report
 
 [testenv:docs]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
@@ -57,6 +61,7 @@ enable-extensions = H106,H203
 exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,tools,.ropeproject,rally-scenarios,neutron/tests/unit/vmware*,releasenotes
 
 [testenv:pylint]
+basepython = python3
 deps =
   {[testenv]deps}
   pylint
@@ -68,9 +73,11 @@ import_exceptions = neutron.i18n
 local-check-factory = neutron_lib.hacking.checks.factory
 
 [testenv:genconfig]
+basepython = python3
 commands = oslo-config-generator --config-file=etc/oslo-config-generator/kuryr.conf
 
 [testenv:releasenotes]
+basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -W -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html