stdlib: Add bytes defaults (#9660)

stdlib/gzip.pyi

@@ -79,9 +79,9 @@ def open(
 
 class _PaddedFile:
     file: _ReadableFileobj
-    def __init__(self, f: _ReadableFileobj, prepend: bytes = ...) -> None: ...
+    def __init__(self, f: _ReadableFileobj, prepend: bytes = b"") -> None: ...
     def read(self, size: int) -> bytes: ...
-    def prepend(self, prepend: bytes = ...) -> None: ...
+    def prepend(self, prepend: bytes = b"") -> None: ...
     def seek(self, off: int) -> int: ...
     def seekable(self) -> bool: ...