Thanks! I recall this person recently having a similar problem; in their case, the solution was to sudo apt-get install python-ndg-httpsclient -t jessie-backports.
I think the problem is that the package has an undeclared dependency - python-requests recommends python-ndg-httpsclient but neither it nor any certbot package explicitly declares an actual dependency on it - which I guess it probably should, if Certbot doesn’t work at all without it.