Support Python3

The previous CL https://crrev.com/c/2796724 passed luci-py's CQ.
But failed on dev.

This CL passes
- high_test.py, parallel_test.py, sign_pythonrsa_test.py, fastboot_test.py in Python3.
- looks working on dev http://screen/7Kfj72L4UfqNGti

Changes
- fixes import errors
- fixes simple syntax errors caught by python-modernize
- changes strings to byte strings everywhere.

Bug: 1012230
Change-Id: I51347399f7e4e8b9bf3a4279f589691600fdab9e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2820831
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Junji Watanabe <jwata@google.com>
20 files changed