[tools] Allow python3 for gm.py

The reason mentioned for requiring python2 is no longer valid.

Bug: chromium:1292013, chromium:1292016
Change-Id: Id8fc938d32c8e967fff74239ccba8ad79e517c57
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3464034Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79118}

tools/dev/gm.py

-#!/usr/bin/env python2
+#!/usr/bin/env python
 # 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.
@@ -21,9 +21,6 @@ Flags are passed unchanged to the test runner. They must start with -- and must
 not contain spaces.
 """
 # See HELP below for additional documentation.
-# Note on Python3 compatibility: gm.py itself is Python3 compatible, but
-# run-tests.py, which will be executed by the same binary, is not; hence
-# the hashbang line at the top of this file explicitly requires Python2.
 
 from __future__ import print_function
 import errno