devmode-test: fix running with Python 3

Use a syntax that works with both versions.

BUG=chromium:998225
TEST=devinstall_test.py passes w/python->python3

Change-Id: Ic54de5418b88cd837ef7469b1431be5374605bad
Reviewed-on: https://chromium-review.googlesource.com/1799784
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>