debian: Add packaging for python3-notmuch2

Ship a new debian package for the notmuch2 CFFI-based Python interface
to notmuch.

Unlike the notmuch python module, the new notmuch2 module is no longer
arch-independent, because it builds and ships a shared object in
addition to the python code.

This patch encourages new downstream development to rely on notmuch2
instead of on notmuch, to get the benefits of the new module.

I welcome any suggested improvements to this packaging, but it appears
to me to be sufficient to get "import notmuch2" to work and do some
basic tests.