In cyclic gc, clear all weakrefs to unreachable objects before allowing any
Python code (weakref callbacks or __del__ methods) to run.
This is a critical bugfix, affecting all versions of Python since weakrefs
were introduced.
Also SF patch 843455.
This is a critical bugfix, backported from 2.4 development.
I don't intend to backport beyond 2.3 maint. The bugs this fixes
have been there since weakrefs were introduced.