Hi, as I stated, the original patch was simply our original implementation.
Here is a new patch.  It is simpler:
1) it exposes a gc.callbacks list where users can register themselves, in the spirit of sys.meta_path
2) One can have multiple callbacks
3) Improve error handling
4) The callback is called with a "phase" argument, currently 0 for start, and 1 for the end.

Let's start bikeshedding the calling signature.  I like having a single callback, since multiple callables are a nuisance to manage.

Once we agree, I'll post a patch for the documentation, and unittests.