Add sys._debugmallocstats (#3401)

stdlib/3/sys.pyi

@@ -128,6 +128,7 @@ version_info: _version_info
 def call_tracing(fn: Callable[..., _T], args: Any) -> _T: ...
 def _clear_type_cache() -> None: ...
 def _current_frames() -> Dict[int, Any]: ...
+def _debugmallocstats() -> None: ...
 def displayhook(value: Optional[int]) -> None: ...
 def excepthook(type_: Type[BaseException], value: BaseException,
                traceback: TracebackType) -> None: ...