CLOS is metacircular, implemented in itself. Moreover, it decouples state (classes and instances) from protocol (generic functions and methods) and through the MOP allows you to reprogram every aspect of its behavior.
Python's object system suffers from the same ailments that Python itself does. It's an amalgamation of various hacks, some obviously inspired by Common Lisp [1], others made up on the spot in order to make everything "fit" together.
In order to best see this, browse CPython's source and find how metaclasses are implemented. Then imagine what you would have to do if you wanted to radically change Python's object system. Contrast with the Common Lisp MOP.
[1] Here is Guido talking about Python's metaobject protocol. Taking into account everything it lacks and how it's implemented, it's not really a metaobject protocol in the CLOS sense, but it's enlightening to read GvR's description of his "aha!" moments in the slides -- which he came upon when he read "The Art of the Metaobject Protocol" -- and then realize that he simply picked very few of the ideas from it whilst completely missing the essence:
So yes, Python has metaclasses but metaclasses are only ~5% of what makes CLOS (and AMOP) iconoclastic. For the rest, read "The Art of the Metaobject Protocol" [2]. This should convince you that Python is really nothing like Lisp and the philosophy of metaprogramming that Lisp espouses is dumbed down and made pretty much powerless (in a similar way, interactive programming) in Python.
CLOS (as well as a lot of other things in Common Lisp, like the error system) depends on CLOS being present. This is why writing a competent CLOS compiler that can bootstrap itself is very difficult. But it's so worth it.
The canonical implementation (PCL) was written in pre-ANSI CL without CLOS. There's a number of things that are difficult there, but can't put my finger on any related to bootstrapping.
Never meant to say python metaclasses were equal to CLOS MOP. But the spirit of programming at the OO metalevel is very much similar to what CLOS is about.