lint python modules as python3

We don't need Python 2 support anymore.

BUG=chromium:1027199
TEST=CQ passes

Change-Id: Ie49e76803bbe6c3a24d7dd85ea40938d4e410097
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2690716
Reviewed-by: Vyshu Khota <vyshu@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index 42156b3..d1420cc 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,5 +1,5 @@
 [Hook Scripts]
-cros lint = cros lint ${PRESUBMIT_FILES}
+cros lint = cros lint --py3 ${PRESUBMIT_FILES}
 gnlint = ../../../platform2/common-mk/gnlint.py ${PRESUBMIT_FILES}
 
 [Hook Overrides]