Update json.pyi (#957)

According to https://docs.python.org/3/library/json.html#json.loads, the s parameter can be "a str, bytes or bytearray instance".

stdlib/3/json.pyi

@@ -31,7 +31,7 @@ def dump(obj: Any,
     sort_keys: bool = ...,
     **kwds: Any) -> None: ...
 
-def loads(s: str,
+def loads(s: Union[str, bytes, bytearray],
     encoding: Any = ...,  # ignored and deprecated
     cls: Any = ...,
     object_hook: Optional[Callable[[Dict], Any]] = ...,