[7.0] Adding l10n it wht vop

Moving from https://code.launchpad.net/~openobject-italia-core-devs/openobject-italia/adding_l10n_it_wht_vop/+merge/212314
For readability, if might help to, instead do:

```python
for voucher in self.browse(cr, uid, ids, context=context):
    if not voucher.withholding_move_ids:
        continue
    ...
```
```