build_class(): one more (hopefully the last) step on the way to backwards compatibility. When using the class of the first base as the metaclass, use its __class__ attribute in preference over its ob_type slot. This ensures that we can still use classic classes as metaclasse, as shown in the original "Metaclasses" essay. This also makes all the examples in Demo/metaclasses/ work again (maybe these should be turned into a test suite?).