activitypub.Collection: is_ordered doesn't work for OrderedCollectionPage

```python
def is_ordered(self):
return 'OrderedCollection' == self._delegate.json_field('type') # <- May also be OrderedCollectionPage

def items(self) -> Iterator[AnyObject]:
items = self._delegate.json_field('orderedItems' if self.is_ordered() else 'items')
# ...
```