Set literal default for bz2.BZ2Compressor.__init__ (#12798)

stdlib/bz2.pyi

@@ -131,7 +131,7 @@ class BZ2File(BaseStream, IO[bytes]):
 
 @final
 class BZ2Compressor:
-    def __init__(self, compresslevel: int = ...) -> None: ...
+    def __init__(self, compresslevel: int = 9) -> None: ...
     def compress(self, data: ReadableBuffer, /) -> bytes: ...
     def flush(self) -> bytes: ...