Support streaming datasets that use pathlib
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I've tried https://github.com/huggingface/datasets/issues/2866 again, and I get the same error.

```python
import datasets as ds
ds.load_dataset('counter', split="train", streaming=False)
``` @severo Issue #2866 is not fully fixed yet: multiple patches need to be implemented for `pathlib`, as that dataset uses quite a lot of `pathlib` functions... 😅  No worry and no stress, I just wanted to check for that case :) I'm very happy that you're working on issues I'm interested in!
This PR extends the support in streaming mode for datasets that use `pathlib.Path`.

Related to: #2866.
CC: @severo 
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{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2874', 'html_url': 'https://github.com/huggingface/datasets/pull/2874', 'diff_url': 'https://github.com/huggingface/datasets/pull/2874.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2874.patch', 'merged_at': '2021-09-07T11:41:15Z'}