[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/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/