tkinter: add _windowingsystem() (#7803)

Author: Akuli

stdlib/tkinter/__init__.pyi

@@ -486,6 +486,8 @@ class Misc:
     def unbind_class(self, className: str, sequence: str) -> None: ...
     def mainloop(self, n: int = ...) -> None: ...
     def quit(self) -> None: ...
+    @property
+    def _windowingsystem(self) -> Literal["win32", "aqua", "x11"]: ...
     def nametowidget(self, name: str | Misc | _tkinter.Tcl_Obj) -> Any: ...
     def register(
         self, func: Callable[..., Any], subst: Callable[..., Sequence[Any]] | None = ..., needcleanup: int = ...