How is one meant to use the collections.abc classes with multiple inheritance?

I'm having an issue where I want to use the mixin methods from collections.abc.MutableSequence, but I also have to inherit from something else.

class Thing(urwid.Pile, collections.abc.MutableSequence):
    ...

I end up with

TypeError: metaclass conflict: the metaclass of a derived class must be
a (non-strict) subclass of the metaclasses of all its bases

How do I determine what's going on, and fix it? metaclass = ABCMeta doesn't do the trick, for what it's worth.