[tools] Use python3 by default for gm.py

Change-Id: I16a090fc9af5447c5d36e7bbd4bece1537724678
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3637792
Auto-Submit: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80451}

tools/dev/gm.py

-#!/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.