Stop using /usr/bin/python directly

Use /usr/bin/env python instead

R=dpranke@google.com

Bug: 1191100
Change-Id: I8de37ea7a02e7236d3bd36d4bae8cf4ffcf75ab9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2780631
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

gerrit_client.py

-#!/usr/bin/python
+#!/usr/bin/env python
 # 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.

recipes/recipe_modules/gitiles/resources/gerrit_client.py

-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright 2014 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.

recipes/recipe_modules/gsutil/resources/gsutil_smart_retry.py

-#!/usr/bin/python
+#!/usr/bin/env python
 #
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be