grit: pak_util: handle Python 3 argparse changes

The required behavior of argparse subparsers changed in Python 3.
Add some code that works in Python 2 & Python 3 to retain the old
behavior.

Bug: 983071
Test: `./pak_util.py` doesn't crash under Python 3
Change-Id: I6820c65e365da662633a2eb4386e4077fedd3174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771983Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690911}

tools/grit/pak_util.py

@@ -100,7 +100,11 @@ def _ListMain(args):
 def main():
   parser = argparse.ArgumentParser(
       description=__doc__, formatter_class=argparse.RawTextHelpFormatter)
-  sub_parsers = parser.add_subparsers()
+  # Subparsers are required by default under Python 2.  Python 3 changed to
+  # not required, but didn't include a required option until 3.7.  Setting
+  # the required member works in all versions (and setting dest name).
+  sub_parsers = parser.add_subparsers(dest='action')
+  sub_parsers.required = True
 
   sub_parser = sub_parsers.add_parser('repack',
       help='Combines several .pak files into one.')