update tox to test st2client under python 3.5

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 # envlist = py26,py27,py34,flake8-py2,flake8-py3,pylint,pylint3
-envlist = py27,flake8-py2
+envlist = py27,flake8-py2,py35
 skipsdist = true
 skip_missing_interpreters=true
 
@@ -26,6 +26,17 @@ commands =
     nosetests -sv st2reactor/tests/unit
     nosetests -sv st2tests/tests/unit
 
+[testenv:py35]
+basepython = python3.5
+setenv = PYTHONPATH = {toxinidir}/external
+         VIRTUALENV_DIR = {envdir}
+install_command = pip install -U --force-reinstall {opts} {packages}
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+       -e{toxinidir}/st2client
+commands =
+    nosetests -sv st2client/tests/unit
+
 [testenv:venv]
 commands = {posargs}