Marshal dumps/loads uses bytes (#3061)

The marshal.dumps and marshal.loads functions should return and accept
bytes, respectively, rather than a string.

stdlib/2and3/marshal.pyi

@@ -4,5 +4,5 @@ version: int
 
 def dump(value: Any, file: IO[Any], version: int = ...) -> None: ...
 def load(file: IO[Any]) -> Any: ...
-def dumps(value: Any, version: int = ...) -> str: ...
-def loads(string: str) -> Any: ...
+def dumps(value: Any, version: int = ...) -> bytes: ...
+def loads(string: bytes) -> Any: ...