Add type stub for sys._is_immortal (#14931)

stdlib/sys/__init__.pyi

@@ -501,3 +501,4 @@ if sys.version_info >= (3, 12):
 if sys.version_info >= (3, 14):
     def is_remote_debug_enabled() -> bool: ...
     def remote_exec(pid: int, script: StrOrBytesPath) -> None: ...
+    def _is_immortal(op: object, /) -> bool: ...