Use PyErr_FormatUnraisable

Author: sergey-miryanov

Python/gc.c

@@ -914,7 +914,8 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old, bool allow_callbacks)
         /* copy-paste of weakrefobject.c's handle_callback() */
         temp = PyObject_CallOneArg(callback, (PyObject *)wr);
         if (temp == NULL) {
-            PyErr_WriteUnraisable(callback);
+            PyErr_FormatUnraisable("Exception ignored on "
+                                   "calling weakref callback %R", callback);
         }
         else {
             Py_DECREF(temp);