[turtle] Use bool for add param

`add` parameter is passed to Tkinter methods that, according to typeshed, also accept `Literal["", "+"]`, but [documentation](docs.python.org/dev/library/turtle.html) only mentions `True` and `False`, so I decided to leave only the bool type.
The `RawTurtle.{onclick,onrelease,ondrag}` methods also only used the bool type before.

Author: donbarbos

stdlib/turtle.pyi

@@ -266,7 +266,7 @@ class TurtleScreen(TurtleScreenBase):
     def window_height(self) -> int: ...
     def getcanvas(self) -> Canvas: ...
     def getshapes(self) -> list[str]: ...
-    def onclick(self, fun: Callable[[float, float], object], btn: int = 1, add: Any | None = None) -> None: ...
+    def onclick(self, fun: Callable[[float, float], object], btn: int = 1, add: bool | None = None) -> None: ...
     def onkey(self, fun: Callable[[], object], key: str) -> None: ...
     def listen(self, xdummy: float | None = None, ydummy: float | None = None) -> None: ...
     def ontimer(self, fun: Callable[[], object], t: int = 0) -> None: ...
@@ -561,7 +561,7 @@ def window_width() -> int: ...
 def window_height() -> int: ...
 def getcanvas() -> Canvas: ...
 def getshapes() -> list[str]: ...
-def onclick(fun: Callable[[float, float], object], btn: int = 1, add: Any | None = None) -> None: ...
+def onclick(fun: Callable[[float, float], object], btn: int = 1, add: bool | None = None) -> None: ...
 def onkey(fun: Callable[[], object], key: str) -> None: ...
 def listen(xdummy: float | None = None, ydummy: float | None = None) -> None: ...
 def ontimer(fun: Callable[[], object], t: int = 0) -> None: ...
@@ -776,8 +776,8 @@ def getturtle() -> Turtle: ...
 
 getpen = getturtle
 
-def onrelease(fun: Callable[[float, float], object], btn: int = 1, add: Any | None = None) -> None: ...
-def ondrag(fun: Callable[[float, float], object], btn: int = 1, add: Any | None = None) -> None: ...
+def onrelease(fun: Callable[[float, float], object], btn: int = 1, add: bool | None = None) -> None: ...
+def ondrag(fun: Callable[[float, float], object], btn: int = 1, add: bool | None = None) -> None: ...
 def undo() -> None: ...
 
 turtlesize = shapesize