modules.mac_user: xrange() => six.moves.range()

```
************* Module salt.modules.mac_user
salt/modules/mac_user.py:58: [W1613(xrange-builtin), _first_avail_uid] xrange built-in referenced
salt/modules/mac_user.py:113: [W1613(xrange-builtin), add] xrange built-in referenced
```