MLECO-4065: Revising scripts to use Python3.9

Revising documentation and scripts to use Python3.9 explicitly
to remove the need for installing it system-wide that might
break a distro's desktop and pacakge manager utilities.

Change-Id: I683b55dd0243d0a726dc94eba2431005d4897c8c
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>