[gm.py] Migrate to Python 3

The code was almost compatible, only one small issue had snuck in.

No-try: true
Change-Id: I52225fb2092bf16a5fffbde957cd1dfe4f2c4fd6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2093492Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66642}

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.
@@ -174,7 +174,7 @@ def _CallWithOutput(cmd):
   try:
     while True:
       try:
-        data = os.read(master, 512)
+        data = os.read(master, 512).decode('utf-8')
       except OSError as e:
         if e.errno != errno.EIO: raise
         break # EIO means EOF on some systems