The argparse Action class omits "required" for the "names" list

This has the user visible effect of the *required* attribute not displaying in the repr:

```
>>> a = ArgumentParser().add_argument('-a', required=True)
>>> a.required
True
>>> a
_StoreAction(option_strings=['-a'], dest='a', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
```
