sizelimit: port to Python 3

In the tests, use bytes for the request body, since that is what is used by
webob in Python 3.

In RequestBodySizeLimiter.__call__(), do not compare NoneType to an integer: it
is forbidden in Python 3. Instead, properly handle the fact that
req.content_lenght might be None.

Change-Id: I179997789b9e2fcd3f3326e4d724e547c807f195
Merge "sizelimit: port to Python 3"