read from cls.__dict__ so init_subclass works

Modified the :class:`.DeclarativeMeta` metaclass to pass ``cls.__dict__``
into the declarative scanning process to look for attributes, rather than
the separate dictionary passed to the type's ``__init__()`` method. This
allows user-defined base classes that add attributes within an
``__init_subclass__()`` to work as expected, as ``__init_subclass__()`` can
only affect the ``cls.__dict__`` itself and not the other dictionary. This
is technically a regression from 1.3 where ``__dict__`` was being used.

Additionally makes the reference between ClassManager and
the declarative configuration object a weak reference, so that it
can be discarded after mappers are set up.

Fixes: #7900
Change-Id: I3c2fd4e227cc1891aa4bb3d7d5b43d5686f9f27c

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