Xapian 2 was released back in March and I'm working towards a transition
to it in Debian.  There are packages in experimental (currently of
2.1.0).

The currently packaged version of python-xapian-haystack isn't
compatible, but the newer upstream version 4.0.0 addresses this
(while maintaining compatibility with Xapian 1.4.x).  Therefore please
update to the latest upstream release.

Cheers,
    Olly