Ubuntu: ensure /usr/bin/python is python3

/usr/bin/python may be python 2 on Focal, which causes problems with
Ansible on the control host. By installing the python-is-python3 package
we ensure that the correct interpreter is used.

This change updates the installation documentation and development
environment scripts.

Story: 2004960
Task: 42579

Change-Id: Ie94099075bae3c491f9cf830c38e6cfc8af605a6