Hello,
It seems like python-acme in Debian Jessie bpo repo was updated recently, which broke some dep-chains. I ran into the same issue nikhilc describes simply doing a routine apt-get upgrade / dist-upgrade. This is what I did to get certbot (re)installed, at least as a temporary solution:
apt-get remove python-acme
wget http://ftp.us.debian.org/debian/pool/main/p/python-acme/python-acme_0.9.3-1~bpo8+1_all.deb
dpkg -i python-acme_0.9.3-1~bpo8+1_all.deb
apt-get install certbot -t jessie-backports
apt-mark hold python-acme
Note that this is a temporary solution and your mileage may vary. After the bpo dep-chain is unbroken w/r/t python-acme, python-certbot, and certbot, you’ll probably want to:
apt-mark unhold python-acme
And reinstall / upgrade as normal.