[Python-Dev] Better module shutdown procedure

The current shutdown code in pythonrun.c zaps module globals by setting them to None (an attempt to break reference cycles). That causes problems since __del__ methods can try to use the globals after they have been set to None. The procedure implemented by http://bugs.python.org/issue812369 seems to be a better idea. References to modules are replaced by weak references and the GC is allowed to cleanup reference cycles. I would like to commit this change but I'm not sure if it is a good time in the development cycle or if anyone has objections to the idea. Please speak up if you have input. Neil