tox.ini: Add py37 and start linting on py3, not py2
[tox]
-envlist = lint, py27, py34, py35, py36
+envlist = lint, py27, py34, py35, py36, py37
[testenv:lint]
-basepython = python2.7
+basepython = python3.7
deps = pylint
commands = pylint watson_developer_cloud test examples
