Use py3 with gerrit_client.py

R=ehmaldonado@chromium.org

Change-Id: I93a4c20121010e8644982aa7910ff41884734f97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2949972
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

gerrit_client.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 2017 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.