Since Debian stretch was released 2017-06-17 it was updated six times to 9.6 on 2018-11-10.
Over the weekend the debian team released the update to the normal
To resume to the normal update path run these commands:
sudo apt update && sudo apt upgrade && sudo apt autoremove sudo apt dist-upgrade
To cleanup the backports run:
sudo rm /etc/apt/sources.list.d/stretch-backports.list
A newer version of certbot is available in debian backports and here is how to enable it.
Applied how to based on general instructions
First we will add support for tls to apt:
sudo apt install apt-transport-https
Then we will add the backports repo to the system:
echo "deb https://debian.ethz.ch/debian stretch-backports main contrib" | sudo tee /etc/apt/sources.list.d/stretch-backports.list
After that we have to fetch the list of available packages:
sudo apt update
Finally we can update from the main package to the backports package:
sudo apt install -t stretch-backports certbot