Fix str(Unauthorized) which is really problematic on python 3 where __unicode__ is never called anymore. This was caused because in PY3, CubicWebException.__str__ was referencing __unicode__ implementation of CubicWebException, not of its subclass that implements it. Fix a flake8 style warning about lambda assignment as a bonus.

.. _req_module:

:mod:`cubicweb.req`
===================

.. automodule:: cubicweb.req

    .. autoexception:: FindEntityError

    .. autoclass:: RequestSessionBase
        :members: