Support Python 3.6, drop support for Python 3.3 (#20)

* Run tests on Python 3.6, too.
* Update to needed versions to run Python 3.6 successfully.
* Drop official support for Python 3.3 due to its sunset in September 2017.

.travis.yml

@@ -2,9 +2,9 @@ language: python
 sudo: false
 python:
     - 2.7
-    - 3.3
     - 3.4
     - 3.5
+    - 3.6
     - pypy
 install:
     - pip install -U setuptools==33.1.1

docs/index.rst

@@ -30,7 +30,7 @@ You should follow these in most cases.
 Python versions
 ---------------
 
-The ZTK supports CPython 2.7, 3.3, 3.4, 3.5 and PyPy.
+The ZTK supports CPython 2.7, 3.4, 3.5, 3.6 and PyPy.
 
 
 Documentation

ztk-versions.cfg

@@ -59,7 +59,7 @@ zope.size = 4.1.0
 zope.structuredtext = 4.1.0
 zope.tal = 4.2.0
 zope.tales = 4.1.1
-zope.testing = 4.5.0
+zope.testing = 4.6.2
 zope.testrunner = 4.7.0
 zope.traversing = 4.1.0
 zope.viewlet = 4.0.0
@@ -114,6 +114,6 @@ collective.recipe.cmd = 0.11
 mr.developer = 1.34
 z3c.checkversions = 0.5
 z3c.recipe.compattest = 1.0
-zc.buildout = 2.9.3
+zc.buildout = 2.9.4
 zc.recipe.egg = 2.0.3
 zc.recipe.testrunner = 2.0.0