[entity] comparing two entities returns NotImplemented
Fixes a test when using some versions of unittest2, where
assertItemsEqual tries a sort and expects TypeError, which it doesn't
get if we directly raise NotImplementedError. The NotImplemented value
is treated specially by the interpreter:
https://docs.python.org/3.4/library/constants.html#NotImplemented
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc