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Extension case:
Extension developers may mess up too, why leave them orphaned?

Doc case:
UGGGHHH

Your patch is just slicker, why dont I delete this patch and
you submit. What do you say?

I have just 2 suggestions:
1. Declaration of register func would better be in pydebug.h
near Py_FatalError.

2. Last one to call register func owns the hook, this is
just expected case so IMHO remove NULL check from register func.