Unwrap Proxy objects when scanning declared_attr

Unwrap Proxy objects when scanning declared_attr

Fixed bug where the declarative scan for attributes would receive the
expression proxy delivered by a hybrid attribute at the class level, and
not the hybrid attribute itself, when receiving the descriptor via the
``@declared_attr`` callable on a subclass of an already-mapped class. This
would lead to an attribute that did not report itself as a hybrid when
viewed within :attr:`.Mapper.all_orm_descriptors`.

Fixes: #4326
Change-Id: I582d03f05c3768b3344f93e3791240e9e69b9d1e