[python3] make sure staticcontroller returns a bytes instance
The ``publish()`` method of controller is supposed to return a
bytes intsance. Returning a text_type instance will crash later
on in the Pyramid response handler when assigning content to
``response.body`` (at least in Python3)::
>>> from pyramid.response import Response
>>> r = Response()
>>> r.body = ''
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/home/adim/.virtualenvs/…/response.py", line 562, in _body__set
raise TypeError(msg)
TypeError: You cannot set Response.body to a text object (use Response.text)