codecs: _buffer_encode returns (bytes, int) (#9587)

Author: adamjorr

stdlib/codecs.pyi

@@ -182,7 +182,7 @@ class BufferedIncrementalEncoder(IncrementalEncoder):
     buffer: str
     def __init__(self, errors: str = "strict") -> None: ...
     @abstractmethod
-    def _buffer_encode(self, input: str, errors: str, final: bool) -> bytes: ...
+    def _buffer_encode(self, input: str, errors: str, final: bool) -> tuple[bytes, int]: ...
     def encode(self, input: str, final: bool = False) -> bytes: ...
 
 class BufferedIncrementalDecoder(IncrementalDecoder):