Be smarter about clearing the weakref lists for instances, instance methods,

and functions: we only need to call PyObject_ClearWeakRefs() if the weakref
list is non-NULL.  Since these objects are common but weakrefs are still
unusual, saving the call at deallocation time makes a lot of sense.

Objects/classobject.c

@@ -585,6 +585,7 @@ instance_dealloc(register PyInstanceObject *inst)
 	extern long _Py_RefTotal;
 #endif
 	_PyObject_GC_UNTRACK(inst);
+	if (inst->in_weakreflist != NULL)
 		PyObject_ClearWeakRefs((PyObject *) inst);
 
 	/* Temporarily resurrect the object. */
@@ -2071,6 +2072,7 @@ static void
 instancemethod_dealloc(register PyMethodObject *im)
 {
 	_PyObject_GC_UNTRACK(im);
+	if (im->im_weakreflist != NULL)
 		PyObject_ClearWeakRefs((PyObject *)im);
 	Py_DECREF(im->im_func);
 	Py_XDECREF(im->im_self);

Objects/funcobject.c

@@ -270,6 +270,7 @@ static void
 func_dealloc(PyFunctionObject *op)
 {
 	_PyObject_GC_UNTRACK(op);
+	if (op->func_weakreflist != NULL)
 		PyObject_ClearWeakRefs((PyObject *) op);
 	Py_DECREF(op->func_code);
 	Py_DECREF(op->func_globals);