Mark some contextlib method arguments as positional-only (#6118)

Author: kbrose

stdlib/contextlib.pyi

@@ -76,7 +76,7 @@ class ExitStack(ContextManager[ExitStack]):
     def __init__(self) -> None: ...
     def enter_context(self, cm: ContextManager[_T]) -> _T: ...
     def push(self, exit: _CM_EF) -> _CM_EF: ...
-    def callback(self, callback: Callable[..., Any], *args: Any, **kwds: Any) -> Callable[..., Any]: ...
+    def callback(self, __callback: Callable[..., Any], *args: Any, **kwds: Any) -> Callable[..., Any]: ...
     def pop_all(self: Self) -> Self: ...
     def close(self) -> None: ...
     def __enter__(self: Self) -> Self: ...
@@ -94,8 +94,8 @@ if sys.version_info >= (3, 7):
         def enter_async_context(self, cm: AsyncContextManager[_T]) -> Awaitable[_T]: ...
         def push(self, exit: _CM_EF) -> _CM_EF: ...
         def push_async_exit(self, exit: _ACM_EF) -> _ACM_EF: ...
-        def callback(self, callback: Callable[..., Any], *args: Any, **kwds: Any) -> Callable[..., Any]: ...
-        def push_async_callback(self, callback: _CallbackCoroFunc, *args: Any, **kwds: Any) -> _CallbackCoroFunc: ...
+        def callback(self, __callback: Callable[..., Any], *args: Any, **kwds: Any) -> Callable[..., Any]: ...
+        def push_async_callback(self, __callback: _CallbackCoroFunc, *args: Any, **kwds: Any) -> _CallbackCoroFunc: ...
         def pop_all(self: Self) -> Self: ...
         def aclose(self) -> Awaitable[None]: ...
         def __aenter__(self: Self) -> Awaitable[Self]: ...