locate automap base in hierarchy directly

locate automap base in hierarchy directly

Fixed issue in automap where calling ``.prepare()`` from one of the mapped
classes would not use the correct base class when automap detected new
tables, instead using the given class, leading to mappers trying to
configure inheritance. While one should normally call ``.prepare()`` from
the base in any case, it shouldn't misbehave that badly when called from a
subclass.

Fixes: #9367
Change-Id: I705d4d939d45af52bc58a74e65994205ab791634