Fix pylint warnings and use python3 in payload_info am: 178375a7d4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2975032

Change-Id: I40c30c96b5a5b241f8145f65ba858b143df521c1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>