exchange: use rich class for sorting clone bundle entries Python 3 removed the "cmp" argument from sorted(). Custom sorting in Python 3 must be implemented with the dunder comparison methods on types and/or with a "key" function. This patch converts our custom "cmp" function to a custom type. The implementation is very similar to functools.cmp_to_key(). However, cmp_to_key() doesn't exist in Python 2, so we can't use it. This was the only use of the "cmp" argument to sorted() in the code base.