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.

.. _cwprops:

The property mecanism
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.. XXX CWProperty and co


Property API
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.. XXX feed me

Registering and using your own property
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.. XXX feed me