Fixed potential memory leak which could occur if an

Fixed potential memory leak which could occur if an
arbitrary number of :func:`.sessionmaker` objects
were created.   The anonymous subclass created by
the sessionmaker, when dereferenced, would not be garbage
collected due to remaining class-level references from the
event package.  This issue also applies to any custom system
that made use of ad-hoc subclasses in conjunction with
an event dispatcher. [ticket:2650]