Add stub for sys._is_interned (#13778)

Author: ZeroIntensity

stdlib/sys/__init__.pyi

@@ -396,6 +396,7 @@ def intern(string: str, /) -> str: ...
 if sys.version_info >= (3, 13):
     def _is_gil_enabled() -> bool: ...
     def _clear_internal_caches() -> None: ...
+    def _is_interned(string: str, /) -> bool: ...
 
 def is_finalizing() -> bool: ...
 def breakpointhook(*args: Any, **kwargs: Any) -> Any: ...