Goes up the mro and looks for the specified field.

def get_field_mro(cls, field_name):
    """Goes up the mro and looks for the specified field."""
    res = set()
    if hasattr(cls, '__mro__'):
        for _class in inspect.getmro(cls):
            values_ = getattr(_class, field_name, None)
            if values_ is not None:
                res = res.union(set(make_list(values_)))
    return res