Migrate touch_firmware_versions.py to Python 3

BUG=chromium:1031760
TEST=python3 touch_firmware_versions.py

Change-Id: I834273e8259b04e119f261791c1dae637546bb2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/1959073
Tested-by: Greg Edelston <gredelston@google.com>
Reviewed-by: Katherine Threlkeld <kathrelkeld@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Greg Edelston <gredelston@google.com>
diff --git a/provingground/touch_firmware_versions.py b/provingground/touch_firmware_versions.py
index 25b76e6..74970f7 100755
--- a/provingground/touch_firmware_versions.py
+++ b/provingground/touch_firmware_versions.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 # Copyright 2016 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
@@ -21,8 +21,6 @@
   - variable usage is ${VAR_NAME}
 """
 
-from __future__ import print_function
-
 import os
 import re
 import subprocess
@@ -275,7 +273,8 @@
   find_output = ''
   cmd = r'find %s -regex .*touch-firmware-.*-[0-9.]+\.ebuild'
   for d in ['private-overlays/', 'overlays/']:
-    find_output += subprocess.check_output((cmd % d).split(' '))
+    find_output += subprocess.check_output((cmd % d).split(' '),
+            universal_newlines=True)
   ebuilds = find_output.split()
   ebuilds.sort()
   for path in ebuilds:
@@ -285,7 +284,8 @@
   find_output = ''
   cmd = 'find %s -regex .*model.dtsi'
   for d in ['private-overlays/', 'overlays/']:
-    find_output += subprocess.check_output((cmd % d).split(' '))
+    find_output += subprocess.check_output((cmd % d).split(' '),
+            universal_newlines=True)
   dtsis = find_output.split()
   for path in dtsis:
     values_list += find_model_firmwares_in_file(path)