restore declared_attr consumption for __table__

restore declared_attr consumption for __table__

Fixed bug in ORM Declarative where the ``__table__`` directive could not be
declared as a class function with :func:`_orm.declared_attr` on a
superclass, including an ``__abstract__`` class as well as coming from the
declarative base itself.  This was a regression since 1.4 where this was
working, and there were apparently no tests for this particular use case.

Fixes: #11509
Change-Id: I82ef0f93d00cb7a43b0b1b16ea28f1a9a79eba3b