Document support for Python 3.6 (#3904)

setup.py

@@ -90,6 +90,7 @@ classes = """
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.4
     Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
     Programming Language :: Python :: Implementation :: CPython
     Programming Language :: Python :: Implementation :: PyPy
     Operating System :: OS Independent