Return the history manager for a given app model.

def get_history_manager_for_model(model):
    """Return the history manager for a given app model."""
    try:
        manager_name = model._meta.simple_history_manager_attribute
    except AttributeError:
        raise NotHistoricalModelError(
            "Cannot find a historical model for {model}.".format(model=model)
        )
    return getattr(model, manager_name)