Por que é isso:

class MyType(type):
    def __init__(cls, name, bases, attrs):
        print 'created', cls
class MyMixin:
    __metaclass__ = MyType
class MyList(list, MyMixin): pass

ok e funciona como esperado:

created <class '__main__.MyMixin'>
created <class '__main__.MyList'>

Mas isso:

class MyType(type):
    def __init__(cls, name, bases, attrs):
        print 'created', cls
class MyMixin:
    __metaclass__ = MyType
class MyObject(object, MyMixin): pass

Não está bem e explode assim ?:

created <class '__main__.MyMixin'>
Traceback (most recent call last):
  File "/tmp/junk.py", line 11, in <module>
    class MyObject(object, MyMixin): pass
TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution
order (MRO) for bases object, MyMixin