python: unpythonic result of Message.get_replies()

[Justus Winter]

The attached patch series fixes this problem. Note that the wrapping
nature of the notmuch bindings makes it kind of awkward to fix the
behavior.

I've decided to avoid introducing code to the Messages class to
indicate that there are no messages and there is no notmuch object
being wrapped, but to subclass it and change the constructor and
__next__ function.

Well, what do you think?
Justus