Dang, thanks for pointing that out. I built this server less than two years ago so I thought I had a couple years to go. I must have decided not to use the later 16.04 for some reason.
Meanwhile, this seems to work to update existing components and get certbot-auto running on Ubuntu 14.04.6:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install python3.4-venv
sudo mv /opt/eff.org/certbot/venv /opt/eff.org/certbot/venv.python27
USE_PYTHON_3=1 /usr/lib/certbot/certbot-auto --version
/usr/lib/certbot/certbot-auto --version
Thanks very much for your help!