_WrappedBuffer protocol isatty() returns bool (#12438)

Fixed new _WrappedBuffer protocol methid `isatty()` correctly returns `bool`

stdlib/io.pyi

@@ -168,7 +168,7 @@ class _WrappedBuffer(Protocol):
     def writable(self) -> bool: ...
     def truncate(self, size: int, /) -> int: ...
     def fileno(self) -> int: ...
-    def isatty(self) -> int: ...
+    def isatty(self) -> bool: ...
     # Optional: Only needs to be present if seekable() returns True.
     # def seek(self, offset: Literal[0], whence: Literal[2]) -> int: ...
     # def tell(self) -> int: ...