tox.ini: Add py37 and start linting on py3, not py2

Author: cclauss

tox.ini

@@ -1,8 +1,8 @@
 [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
 
@@ -14,4 +14,4 @@ commands =
 deps =
      -r{toxinidir}/requirements.txt
      -r{toxinidir}/requirements-dev.txt
-exclude = .venv,.git,.tox,docs
+exclude = .venv,.git,.tox,docs