the-littlest-jupyterhub: Bug Report: Can't install

I wanted to update to the current TLJH version, so I created a new Ubuntu 18.04 live server install, and then tried:

curl https://raw.githubusercontent.com/jupyterhub/the-littlest-jupyterhub/master/bootstrap/bootstrap.py \
| sudo python3 - \
 --admin <my_username>

but the result was

Checking if TLJH is already installed...
Setting up hub environment
Traceback (most recent call last):
  File "<stdin>", line 83, in <module>
  File "<stdin>", line 46, in main
  File "/usr/lib/python3.6/subprocess.py", line 336, in check_output
    **kwargs).stdout
  File "/usr/lib/python3.6/subprocess.py", line 418, in run
    output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['apt-get', 'install', '--yes', 'python3', 'python3-venv']' returned non-zero exit status 100.

I checked, and doing

sudo apt-get update
sudo apt-get install --yes python3-venv

tells me

Package python3-venv is not available, but is referred to by another package.

I assumed venv might be installed already and disabled the line in the bootstrap.py file, but it is definitely not installed nor is any obvious variation available on apt-get list.