bpo-36829: Add _PyErr_WriteUnraisableMsg() (GH-13488)

* sys.unraisablehook: add 'err_msg' field to UnraisableHookArgs.
* Use _PyErr_WriteUnraisableMsg() in _ctypes _DictRemover_call()
  and gc delete_garbage().
diff --git a/Include/cpython/pyerrors.h b/Include/cpython/pyerrors.h
index de6548d..6b0cced 100644
--- a/Include/cpython/pyerrors.h
+++ b/Include/cpython/pyerrors.h
@@ -171,6 +171,9 @@
     const char *reason          /* UTF-8 encoded string */
     );
 
+PyAPI_FUNC(void) _PyErr_WriteUnraisableMsg(
+    const char *err_msg,
+    PyObject *obj);
 
 #ifdef __cplusplus
 }