Remove false inheritance from io.IncrementalNewlineDecoder (#12871)

Author: tungol

stdlib/_io.pyi

@@ -181,11 +181,13 @@ class StringIO(TextIOWrapper, TextIOBase, _TextIOBase):  # type: ignore[misc]  #
     name: Any
     def getvalue(self) -> str: ...
 
-class IncrementalNewlineDecoder(codecs.IncrementalDecoder):
+class IncrementalNewlineDecoder:
     def __init__(self, decoder: codecs.IncrementalDecoder | None, translate: bool, errors: str = ...) -> None: ...
     def decode(self, input: ReadableBuffer | str, final: bool = False) -> str: ...
     @property
     def newlines(self) -> str | tuple[str, ...] | None: ...
+    def getstate(self) -> tuple[bytes, int]: ...
+    def reset(self) -> None: ...
     def setstate(self, state: tuple[bytes, int], /) -> None: ...
 
 if sys.version_info >= (3, 10):