py/runtime: Allow types to use both .attr and .locals_dict.

Make it possible to proceed to a regular lookup in locals_dict if the
custom type->attr fails.  This allows type->attr to extend rather than
completely replace the lookup in locals_dict.

This is useful for custom builtin classes that have mostly regular methods
but just a few special attributes/properties.  This way, type->attr needs
to deal with the special cases only and the default lookup will be used for
generic methods.

Signed-off-by: Laurens Valk <laurens@pybricks.com>

py/obj.h

@@ -551,6 +551,7 @@ struct _mp_obj_type_t {
     //
     // dest[0] = MP_OBJ_NULL means load
     //  return: for fail, do nothing
+    //          for fail but continue lookup in locals_dict, dest[1] = MP_OBJ_SENTINEL
     //          for attr, dest[0] = value
     //          for method, dest[0] = method, dest[1] = self
     //