Migrate CheckAVDevices.py to Python 3

Note: The script does not currently run successfully in master.

BUG=chromium:1031760
TEST=None

Change-Id: Id588b86215ea83229e2af48040dda4685a785140
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1959075
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Aashutosh Kalyankar <aashutoshk@chromium.org>
Commit-Queue: Greg Edelston <gredelston@google.com>
1 file changed