Add a structseq class to _typeshed (#6560)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stdlib/signal.pyi

@@ -1,7 +1,9 @@
 import sys
+from _typeshed import structseq
 from enum import IntEnum
 from types import FrameType
 from typing import Any, Callable, Iterable, Optional, Tuple, Union
+from typing_extensions import final
 
 NSIG: int
 
@@ -129,8 +131,8 @@ else:
         SIGPWR: Signals
         SIGRTMAX: Signals
         SIGRTMIN: Signals
-        class struct_siginfo(Tuple[int, int, int, int, int, int, int]):
-            def __init__(self, sequence: Iterable[int]) -> None: ...
+        @final
+        class struct_siginfo(structseq[int], Tuple[int, int, int, int, int, int, int]):
             @property
             def si_signo(self) -> int: ...
             @property