fcntl: fix fcntl arg type (s#) (#9052)

Author: JelleZijlstra

stdlib/fcntl.pyi

@@ -103,7 +103,7 @@ if sys.platform != "win32":
     @overload
     def fcntl(__fd: FileDescriptorLike, __cmd: int, __arg: int = ...) -> int: ...
     @overload
-    def fcntl(__fd: FileDescriptorLike, __cmd: int, __arg: bytes) -> bytes: ...
+    def fcntl(__fd: FileDescriptorLike, __cmd: int, __arg: str | ReadOnlyBuffer) -> bytes: ...
     @overload
     def ioctl(__fd: FileDescriptorLike, __request: int, __arg: int = ..., __mutate_flag: bool = ...) -> int: ...
     @overload