Reintroduce ReadableBuffer in builtins so the bug can be reproduced without using stubtest

Author: AlexWaygood

stdlib/builtins.pyi

@@ -7,6 +7,7 @@ from _typeshed import (
     OpenBinaryModeUpdating,
     OpenBinaryModeWriting,
     OpenTextMode,
+    ReadableBuffer,
     Self,
     StrOrBytesPath,
     SupportsAnext,
@@ -79,7 +80,6 @@ BufferedWriter = Any
 FileIO = Any
 CodeType = Any
 TracebackType = Any
-ReadableBuffer = Any
 
 class _SupportsIter(Protocol[_T_co]):
     def __iter__(self) -> _T_co: ...