This patches changes PyImport_Cleanup() in an attempt
to make the module shutdown order more predictable in
the presence of modules that import each other. Before
it explicitely clears the globals of the modules, it
relies on the GC to try to do it more cleanly.

http://mail.python.org/pipermail/python-dev/2003-September/038238.html

To prevent objects with __del__ methods from keeping
whole modules alive I actually replace each module with
a weak reference to it in sys.modules. This allows me
to find modules that remain alive after a call to
PyGC_Collect(), and then go back to the old technique
of clearing their globals.

Note that weak references to dead cycles containing
objects with finalizers have a strange property in
Python: if you use the weak reference again you can
break the cycles, but the objects with finalizers still
won't be collected because they are in gc.garbage. As
this is exactly what occurs above, I clear the
gc.garbage list explicitely before the final
PyGC_Collect() call. I'm not sure exactly what this
might do; could it release older objects that were
never put in a module but that at some time were put in
gc.garbage and whose cycles were later broken? If so,
is it a good thing to release them now? (Would it make
sense to clear gc.garbage and call gc.collect again
from time to time to check if the objects are still in
a cycle?)

This patch does not change the behavior of module
objects clearing their globals dictionary as soon as
they are deallocated. This could be work investigating.

This patch puts weak references (actually proxies) in
sys.modules but only at shutdown time. Moure thoughts
could be given towards allowing weak references during
normal program execution as well. To do so we must
ensure that 'import' statements return the real module,
not the weak proxy object, which is more difficult than
it first seems in the presence of packages.

And finally -- this patch has not really been tested,
apart from the fact that it passes the test suite.