Require Python 3.3 and higher for extensions

Flask and several extensions already supports Python 3.3 and higher. By
requiring approved extensions to support Python 3.3 as well we can
quickly achieve better Python 3 adoption and make using Python 3 easier
for users.

The effort of supporting both Python 2.7 and Python 3.3 is small enough
that it shouldn't be a problem to require this from extension authors.

docs/extensiondev.rst

@@ -386,7 +386,7 @@ extension to be approved you have to follow these guidelines:
    themes for Sphinx documentation.
 8. The ``zip_safe`` flag in the setup script must be set to ``False``,
    even if the extension would be safe for zipping.
-9. An extension currently has to support Python 2.7.
+9. An extension currently has to support Python 2.7, Python 3.3 and higher.
 
 
 .. _ext-import-transition: