Switch gn wrapper scripts to invoke python3.

Bug: 941669
Change-Id: I515459793f2a406a959c361f9884b183e21ebdf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2825439
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>

gn

@@ -5,4 +5,4 @@
 
 base_dir=$(dirname "$0")
 
-PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/gn.py" "$@"
+PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/gn.py" "$@"

gn.bat

@@ -9,4 +9,4 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python "%~dp0\gn.py" %*
+python3 "%~dp0\gn.py" %*

gn.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2013 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.