[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.

Pygments
#fyzz XXX pip install fails
cubicweb-card
cubicweb-file
cubicweb-localperms
cubicweb-tag