Creating weak references from extension module

[Brian Quinlan]

It seems like the weak reference module is less accessible from C than
it should be. I've been trying to figure out a good way to create weak
references from an extension module and it's not obvious. weakref_proxy
and weakref_ref are static so they are no goes unless I call them
through Python. 

Any reason not to add a nice function to _weakref.c that allows you to
construct weak proxies and weak references?

Cheers,
Brian