by default python-pip doesn't exist in Ubuntu, change to python3-pip

[sadreck]

To be honest the python-pip works just fine for 14/16/18, but not for 20.04. Tried python3-pip on 16/18 and it worked fine too.

I'll try to find some time to test the setup on 20.04, but until then I'll merge this.

Thanks for your help, Pavel