Thanks for the review, Nick.  I'll be uploading a new patch in a couple hours with your recommended fixes.

Regarding the comments on python-ideas, would it be better to use a weakref proxy around the function, to help with the reference cycles?  That's what I was doing with my closure-based solution.  I didn't do it here just to see what would happen and I didn't see any problems in my very limited testing (basically just 'make test').  I don't mind using weakrefs and, if it matters, I could pre-allocate the weakref proxy in PyFunction_New to save a little overhead at each call. 

For the moment I left in the code to limit f_func to only functions.  I'll respond to that on python-ideas.

Finally, how does this patch relate to the ABI?  I'm not too familiar with it (read through PEP 384) and want to make sure I'm okay here.