Make update_device.py script compatible with python3 am: 4b883ea0ac am: 1cbf618b1f am: 4c859bfff6

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1455255

Change-Id: I381998b62ee3d091b76f2ddf0397c2162d4eab66