Timestamp not returned as datetime objects in streaming mode

As reported in (internal) https://github.com/huggingface/datasets-server/issues/397

```python
>>> from datasets import load_dataset
>>> dataset = load_dataset("ett", name="h2", split="test", streaming=True)
>>> d = next(iter(dataset))
>>> d['start']
Timestamp('2016-07-01 00:00:00')
```

while loading in non-streaming mode it returns `datetime.datetime(2016, 7, 1, 0, 0)`