Explicitly run everything with python3

R=aravindvasudev@google.com, gavinmak@google.com

Change-Id: Iaa5e960c6226dea3a0814c7cb778d952eafb1502
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3525960
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/metrics_utils.py b/metrics_utils.py
index 96982a9..e1757bc 100644
--- a/metrics_utils.py
+++ b/metrics_utils.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2018 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.