[gm.py] Specify python2 for now

Because run-tests.py still requires it.

No-try: true
Change-Id: Ief1f3d7a93ba4c36232420ee9ab0a4ff3ea6739b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2096628
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66664}

tools/dev/gm.py

-#!/usr/bin/env python3
+#!/usr/bin/env python2
 # 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.
@@ -18,6 +18,9 @@ All arguments are optional. Most combinations should work, e.g.:
     gm.py x64 mjsunit/foo cctest/test-bar/*
 """
 # 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