Function _PyWeakref_ClearRef

Cyclic gc uses this to *just* clear the passed-in reference, leaving * the callback intact and uncalled. It must be possible to call self's * tp_dealloc() after calling this, so self has to be left in a sane enough * state for that to work. We expect tp_dealloc to decref the callback * then. The reason for not letting clear_weakref() decref the callback * right now is that if the callback