bpo-26510: make argparse subparsers required by default (#3027)

This fixes a regression from Python 2.  To get optional subparsers,
use the new parameter ``add_subparsers(required=False)``.

Patch by Anthony Sottile.

Lib/argparse.py

@@ -1066,6 +1066,7 @@ class _SubParsersAction(Action):
                  prog,
                  parser_class,
                  dest=SUPPRESS,
+                 required=True,
                  help=None,
                  metavar=None):
 
@@ -1079,6 +1080,7 @@ class _SubParsersAction(Action):
             dest=dest,
             nargs=PARSER,
             choices=self._name_parser_map,
+            required=required,
             help=help,
             metavar=metavar)