base all_orm_descriptors ordering on cls.__dict__ + cls.__mro__

base all_orm_descriptors ordering on cls.__dict__ + cls.__mro__

Adjusted the workings of the :meth:`_orm.Mapper.all_orm_descriptors`
accessor to represent the attributes in the order that they are located in
a deterministic way, assuming the use of Python 3.6 or higher which
maintains the sorting order of class attributes based on how they were
declared.   This sorting is not guaranteed to match the declared order of
attributes in all cases however; see the method documentation for the exact
scheme.

Fixes: #5494
Change-Id: I6ee8d4ace3eb8b3f7c9c0f2a3d7e27b5f62abfd3
(cherry picked from commit 9a3fee2cb6b608eb5c0263cf5a7e9085f74f2e73)