/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