tools: run fetch_deps.py with Python 3

Python 3 is now required to run `gclient`. Run `tools/v8/fetch_deps.py`,
which spawns `glient`, with Python 3.

PR-URL: #48729
Fixes: #48728
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Debadree Chatterjee <debadree333@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

Author: richardlau

tools/v8/fetch_deps.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2017 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.

tools/v8/node_common.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2017 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.