Register classes as plugins that are not subclassed from
        IPlugin.
        `classes` may be a single object or an iterable.

def register_classes(self, classes):
        """
        Register classes as plugins that are not subclassed from
        IPlugin.
        `classes` may be a single object or an iterable.
        """
        classes = util.return_list(classes)
        for klass in classes:
            IPlugin.register(klass)