Issue: HANS dataset preview broken

Comment:
The dataset cannot be loaded, be it in normal or streaming mode.

```python
>>> import datasets
>>> dataset=datasets.load_dataset("hans", split="train", streaming=True)
>>> next(iter(dataset))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/iterable_dataset.py", line 497, in __iter__
    for key, example in self._iter():
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/iterable_dataset.py", line 494, in _iter
    yield from ex_iterable
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/iterable_dataset.py", line 87, in __iter__
    yield from self.generate_examples_fn(**self.kwargs)
  File "/home/slesage/.cache/huggingface/modules/datasets_modules/datasets/hans/1bbcb735c482acd54f2e118074b59cfd2bf5f7a5a285d4d540d1e632216672ac/hans.py", line 121, in _generate_examples
    for idx, line in enumerate(open(filepath, "rb")):
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/fsspec/spec.py", line 1595, in __next__
    out = self.readline()
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/fsspec/spec.py", line 1592, in readline
    return self.readuntil(b"\n")
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/fsspec/spec.py", line 1581, in readuntil
    self.seek(start + found + len(char))
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/fsspec/implementations/http.py", line 676, in seek
    raise ValueError("Cannot seek streaming HTTP file")
ValueError: Cannot seek streaming HTTP file
>>> dataset=datasets.load_dataset("hans", split="train", streaming=False)
Downloading and preparing dataset hans/plain_text (download: 29.51 MiB, generated: 30.34 MiB, post-processed: Unknown size, total: 59.85 MiB) to /home/slesage/.cache/huggingface/datasets/hans/plain_text/1.0.0/1bbcb735c482acd54f2e118074b59cfd2bf5f7a5a285d4d540d1e632216672ac...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/load.py", line 1687, in load_dataset
    builder_instance.download_and_prepare(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/builder.py", line 605, in download_and_prepare
    self._download_and_prepare(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/builder.py", line 1104, in _download_and_prepare
    super()._download_and_prepare(dl_manager, verify_infos, check_duplicate_keys=verify_infos)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/builder.py", line 694, in _download_and_prepare
    self._prepare_split(split_generator, **prepare_split_kwargs)
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/datasets/builder.py", line 1087, in _prepare_split
    for key, record in logging.tqdm(
  File "/home/slesage/hf/datasets-preview-backend/.venv/lib/python3.9/site-packages/tqdm/std.py", line 1180, in __iter__
    for obj in iterable:
  File "/home/slesage/.cache/huggingface/modules/datasets_modules/datasets/hans/1bbcb735c482acd54f2e118074b59cfd2bf5f7a5a285d4d540d1e632216672ac/hans.py", line 121, in _generate_examples
    for idx, line in enumerate(open(filepath, "rb")):
ValueError: readline of closed file
```