io, codecs: improve bytes handling (#9059)

stdlib/io.pyi

@@ -190,7 +190,7 @@ class StringIO(TextIOWrapper):
 
 class IncrementalNewlineDecoder(codecs.IncrementalDecoder):
     def __init__(self, decoder: codecs.IncrementalDecoder | None, translate: bool, errors: str = ...) -> None: ...
-    def decode(self, input: bytes | str, final: bool = ...) -> str: ...
+    def decode(self, input: ReadableBuffer | str, final: bool = ...) -> str: ...
     @property
     def newlines(self) -> str | tuple[str, ...] | None: ...
     def setstate(self, __state: tuple[bytes, int]) -> None: ...