strip_package: Run on Python 3

This script has few dependencies and can easily run on Python 3 with
minor cleanups.

BUG=chromium:998225, chromium:1114277
TEST=`builder_test.py`

Change-Id: Ib01f1361eebf8c010bb079d8e50902db9e2c1ccf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2350119
Reviewed-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
2 files changed