python-paramiko not available on ubuntu 20

[petersilva]

Just noticed that there is no python2 library for paramiko in ubuntu 20.
Without that dependency, there is no way install sundew on ubuntu >=20.04

So hard stop on Sundew at 18.04. Unless we can build the dependency ourselves... unclear.

[petersilva]

@habilinour @junhu3 @kurt2x