py/objstr: format: Return bytes result for bytes format string.

This is an improvement over previous behavior when str was returned for
both str and bytes input format.  This new behaviour is also consistent
with how the % operator works, as well as many other str/bytes methods.

It should be noted that it's not how current versions of CPython work,
where there's a gap in the functionality and bytes.format() is not
supported.

py/objstr.c

@@ -1387,7 +1387,7 @@ mp_obj_t mp_obj_str_format(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs
     GET_STR_DATA_LEN(args[0], str, len);
     int arg_i = 0;
     vstr_t vstr = mp_obj_str_format_helper((const char*)str, (const char*)str + len, &arg_i, n_args, args, kwargs);
-    return mp_obj_new_str_from_vstr(&mp_type_str, &vstr);
+    return mp_obj_new_str_from_vstr(mp_obj_get_type(args[0]), &vstr);
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(str_format_obj, 1, mp_obj_str_format);