1) Do not use gc.get_referrers as this is highly inefficient.
(the returned list contains a lot of objects, some of them
are persistent objects, some of them are not classes, etc, etc...
But worse, "dead" objects with a zero reference count can be returned
by this function, and this function would "ressucitate" them.
gc.get_referrers for application code is usually a bad idea)

2) Use instead a Python metaclass to track who's been subclassing
a portal type class, and after a change on the portal type class,
we just call the efficient PortalTypeMetaClass.getSubclassList to
get descendants.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@39751 20353a03-c40f-0410-a6d1-a30d3c3de9de