Add sys.__unraisablehook__ (#7403)

Docs [here](https://docs.python.org/3/library/sys.html#sys.__breakpointhook__). The similar functions `sys.__displayhook__` and `sys.__breakpointhook__` are already included in the stub.

stdlib/sys.pyi

@@ -301,6 +301,7 @@ if sys.version_info >= (3, 8):
         err_msg: str | None
         object: _object | None
     unraisablehook: Callable[[UnraisableHookArgs], Any]
+    def __unraisablehook__(__unraisable: UnraisableHookArgs) -> Any: ...
     def addaudithook(hook: Callable[[str, tuple[Any, ...]], Any]) -> None: ...
     def audit(__event: str, *args: Any) -> None: ...