Fix pyhton3 incompatibilities

This CL also tries to keep consistency with the google/python-adb as much as possible.
https://github.com/google/python-adb

Bug: 1198820
Change-Id: I0c1a9ef4912d5c20afb25467668f7e3f350c4b14
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2929918
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Junji Watanabe <jwata@google.com>