add __init__ to DeclarativeBase directly

add __init__ to DeclarativeBase directly

Fixed regression in :class:`.DeclarativeBase` class where the registry's
default constructor would not be applied to the base itself, which is
different from how the previous :func:`_orm.declarative_base` construct
works. This would prevent a mapped class with its own ``__init__()`` method
from calling ``super().__init__()`` in order to access the registry's
default constructor and automatically populate attributes, instead hitting
``object.__init__()`` which would raise a ``TypeError`` on any arguments.

This is a very simple change in code, however explaining it is
very complicated.

Fixes: #9171
Change-Id: I4baecdf671861a8198d835e286fe19a51ecda126