Python3 cffi bindings

[David Bremner]

Floris Bruynooghe <flub at devork.be> writes:

> IIRC this probably wants to be renamed to "notmuch2" instead of
> "notdb".  Otherwise I'm pretty sure this doesn't cover all the
> current features either.

Yes, I think notmuch2 was the least disliked suggestion. 

> So maybe this can be used as a start to figure out how to merge
> this if there's still an interest in this.

I'm interested. The blocker for me at the moment is getting the tests
working without tox / venvs.  I'm hoping we can test with the system
python and the built, but not installed module. I guess we need to build
it so that the C extension part is loaded. The shim in
T391-python-cffi.sh doesn't work for me, it doesn't manage to set
PYTHONPATH so that notdb is importable.