[py3k] introduce cubicweb._ (related to #7589459) The end goal is to stop relying on '_' being in the builtin scope. In the mean time, deprecate the builtin '_' so that consumers are warned.

usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py