Limit dc field logic to only fields that are definitely dc

Limit dc field logic to only fields that are definitely dc

Fixed regression where recent changes to support Python dataclasses had the
inadvertent effect that an ORM mapped class could not successfully override
the ``__new__()`` method.

In this case the "__new__" method comes out as staticmethod in
cls.__dict__ vs. a function in the metaclass dict_, so comparing
using identity fails.   I was hoping not to have too much
"dataclass" hardcoded, the logic here if it were generalized
to other attribute declaration systems there
would still have a flag that indicates an attribute is part of
the "special declaration system".

Fixes: #6331
Change-Id: Ia28a44fb57c668fa2fc5cd1ff38fd511f2c747e6