chromeos-config: run all unittests under python3 too

Hopefully prevent regressions in here.

BUG=chromium:999879
TEST=tests pass w/python3

Change-Id: I71870b17f4c5c04d7be3990919901597ed73dea1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1882483
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ned Nguyen <nednguyen@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/chromeos-config/run_tests.sh b/chromeos-config/run_tests.sh
index 1781e93..b80677f 100755
--- a/chromeos-config/run_tests.sh
+++ b/chromeos-config/run_tests.sh
@@ -6,3 +6,4 @@
 # Script to run all Python unit tests in cros_config.
 
 python2 -m unittest discover -p '*test.py'
+python3 -m unittest discover -p '*test.py'