py: Make sure getattr() works with non-interned strings (by interning them).

py/obj.h

@@ -468,6 +468,7 @@ uint mp_obj_str_get_len(mp_obj_t self_in);
 qstr mp_obj_str_get_qstr(mp_obj_t self_in); // use this if you will anyway convert the string to a qstr
 const char *mp_obj_str_get_str(mp_obj_t self_in); // use this only if you need the string to be null terminated
 const char *mp_obj_str_get_data(mp_obj_t self_in, uint *len);
+mp_obj_t mp_obj_str_intern(mp_obj_t str);
 void mp_str_print_quoted(void (*print)(void *env, const char *fmt, ...), void *env, const byte *str_data, uint str_len);
 
 #if MICROPY_PY_BUILTINS_FLOAT