ImmutableFoo can't be a subclass of Foo, since it loses the mutator methods. But nor can Foo be a subclass of ImmutableFoo, since it loses the axiom of immutability (e.g. thread-safety) that ImmutableFoo has.

When you interpret Liskov substitution properly, it's very rare that anything Liskov-substitutes anything, making the entire property meaningless. So just do things based on what works best in the real world and aim for as much Liskov-substitution as is reasonable. Python is duck-typed anyway.

It's a decent guiding principle - Set and ImmutableSet are more substitutable than Set and Map, so Set deriving from ImmutableSet makes more sense than Set deriving from Map. It's just not something you can ever actually achieve.