apply 2to3 to for some Python scripts

Change-Id: I8a8fc9814c3e714bd949480ce0f1d0902554d194
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16560
Reviewed-by: Petr Hosek <phosek@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
diff --git a/tools/find_unreachable.py b/tools/find_unreachable.py
index 406a312..ca8b34b 100755
--- a/tools/find_unreachable.py
+++ b/tools/find_unreachable.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2020 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -71,7 +71,7 @@
   result = find_source_targets_from(unreachable, targets_graph) \
     if cmd_args.no_refs else unreachable
 
-  print '\n'.join(sorted(result))
+  print('\n'.join(sorted(result)))
 
 
 if __name__ == '__main__':