Consult is_attrbute flag to determine descriptor; enable for assoc proxy

Consult is_attrbute flag to determine descriptor; enable for assoc proxy

Fixed bug where the :attr:`.Mapper.all_orm_descriptors` accessor would
return an entry for the :class:`.Mapper` itself under the declarative
``__mapper___`` key, when this is not a descriptor.  The ``.is_attribute``
flag that's present on all :class:`.InspectionAttr` objects is now
consulted, which has also been modified to be ``True`` for an association
proxy, as it was erroneously set to False for this object.

Fixes: #4729
Change-Id: Ia02388cc25d004e32d337140b62a587f3e5a0b7b