backport stable branch and some vreg cleanups: * move initialization_completed from cwvreg to base VRegistry class allowing simplification of CWVregistry * cleanup initialization process: __registered__ is now called after initialization completed, by the relevant registry. * fix/remove deprecated tests

"""pre 3.6 bw compat"""
# pylint: disable-msg=W0614,W0401
from warnings import warn
warn('moved to cubicweb.mail', DeprecationWarning, stacklevel=2)
from cubicweb.mail import *