Put lines 213 and 214 in an else block. I would even keep the most likely case in the if part:

if os_platform not in OLD_UBUNTU_DISTROS:
    mirror = 'http://us.archive.ubuntu.com/ubuntu'
    updates_mirror = "deb http://us.archive.ubuntu.com/ubuntu/ %s-updates main restricted universe multiverse" % (os_platform)
else:
    mirror = 'http://old-releases.ubuntu.com/ubuntu'
    updates_mirror = "deb http://old-releases.ubuntu.com/ubuntu/ %s-updates main restricted universe multiverse" % (os_platform)