PHP 8.1 Deprecation: Return type of DusanKasan\Knapsack\Collection::getIterator()

Running PHP 8.1 the following info is logged:

```
Deprecated: Return type of DusanKasan\Knapsack\Collection::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
```