Avoid unhelpful messages with old Python versions.
Without this change gand using Python 2.6.4 get:

	TypeError: Error when calling the metaclass bases
	    __init__() keywords must be strings

With traceback enabled get:

    django\db\models\base.py .... in __new__
            new_class.add_to_class('_meta', Options(meta, **kwargs))
	        TypeError: Error when calling the metaclass bases
		        __init__() keywords must be strings