certbot-auto is being deprecated upstream. So bench needs to be updated to install certbot instead.
On Ubuntu 20.04 onwards, it is recommended to use the snap instead of the apt package or the manual download.
After using certbot, I ended up using the following commands:
sudo bench setup lets-encrypt example.com
which crashes with the following error:
Reading state information... Done
Note, selecting 'python-is-python2' instead of 'python'
Note, selecting 'python-dev-is-python2' instead of 'python-dev'
Package python-virtualenv is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'python-virtualenv' has no installation candidate
You might need to comment out the authenticator entry in /etc/letsencrypt/configs/example.com.cfg before continuing to generate the certifcates by adding DNS entries.
Thanks for this but it seems your solution only helps to download certbot. The setup however still throws an error in Ubuntu 20.04. The steps mentioned by @idlethread above are recommended for those using Ubuntu 20.04+