python CFFI bindings integration into notmuch build/test

[David Bremner]

David Bremner <david at tethera.net> writes:

> This series is on top of a slightly modified version [1] of
>
>      id:20191008210312.20685-2-flub at devork.be
>
> If you want to try out the new bindings, you can try the branch wip/cffi in the repo
> https://git.notmuchmail.org/git/notmuch
>
> One current annoyance is that the version of pytest on travis is too
> old for what is specified in tox.ini.
>

I fixed this by moving the travis env to bionic (2018.x).  This involved
dropping dkg's notmuch ppa. Daniel, do you happen to know if there is
anything in that repo that we should still keep when building in ubuntu
bionic? FWIW, the build succeeds without that PPA.

d