make anno-only Mapped[] column available for mixins

make anno-only Mapped[] column available for mixins

Documentation is relying on the recently improved
behavior of produce_column_copies() to make sure everything is
available on cls for a declared_attr.  therefore for anno-only
attribute, we also need to generate the mapped_column() up front
before scan is called.

noticed in pylance, allow @declared_attr to recognize
@classmethod also which allows letting typing tools know
something is explicitly a classmethod

Change-Id: I07ff1a642a75679f685914a33c674807929f4918