py: Give up and make mp_obj_str_get_data() deal with bytes too.

This is not fully correct re: error handling, because we should check that
that types are used consistently (only str's or only bytes), but magically
makes lot of functions support bytes.

py/objstr.c

@@ -1647,7 +1647,7 @@ const char *mp_obj_str_get_str(mp_obj_t self_in) {
 }
 
 const char *mp_obj_str_get_data(mp_obj_t self_in, uint *len) {
-    if (MP_OBJ_IS_STR(self_in)) {
+    if (is_str_or_bytes(self_in)) {
         GET_STR_DATA_LEN(self_in, s, l);
         *len = l;
         return (const char*)s;