nebraska: fix unittests under python3

BUG=chromium:1031550
TEST=`python3 ./nebraska_unittest.py` passes

Exempt-From-Owner-Approval: Amin will forgive me i promise
Change-Id: I3592c16d938e81357dfe03bcbe984e98f4065793
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1954679
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed