[gs_archive_server] convert gs_archive_server source files to py3

BUG=chromium:1123908
TEST=Manually tested on chromeos2-devservertest

Change-Id: Idd5c9bcc142a6f11a7a95e8f1feb9b253adc78b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2720705
Tested-by: Sanika Kulkarni <sanikak@chromium.org>
Auto-Submit: Sanika Kulkarni <sanikak@chromium.org>
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
9 files changed