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.

[test] avoid using a deprecated feature of yams handle_file() now require a module name (yams@d9120d7)

[test] add some tests to web.views.baseviews