Support Ubuntu Focal 20.04

[hydro-b]

Ubuntu Focal 20.04 won't ship with python 2 anymore. Current one-context depends on python, which is not available, python2-minimal is available but it would be best not to depend on a deprecated (EOL) python version anymore. What python2 specific functionality is need in one-context?

[vholer]

Looks like just some legacy dependency, I have removed it. Thanks.

We'll release updated packages before Focal gets stable release.