run unittests through py2 & py3

This should help keep from regressing.

BUG=chromium:1003955
TEST=`repo upload` works

Change-Id: I2af2b3c03f88163913c19e3e018f949ed7c76b86
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/repohooks/+/1875509
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 6dd237a..f169d57 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -3,7 +3,8 @@
 
 [Hook Scripts]
 cros lint = cros lint ${PRESUBMIT_FILES}
-pre-upload_unittest = ./pre-upload_unittest.py
+pre-upload_unittest py2 = python2 ./pre-upload_unittest.py
+pre-upload_unittest py3 = python3 ./pre-upload_unittest.py
 
 [Hook Overrides]
 cros_license_check: true