Support streaming datasets that use pathlib
COLLABORATOR
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)
```
2021-09-07T12:50:38Z
https://github.com/huggingface/datasets/pull/2874#issuecomment-914279362
https://api.github.com/repos/huggingface/datasets/issues/2874
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2021-09-07T12:54:43Z
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@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... 😅 
2021-09-07T18:20:54Z
https://github.com/huggingface/datasets/pull/2874#issuecomment-914522713
https://api.github.com/repos/huggingface/datasets/issues/2874
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COLLABORATOR
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!
2021-09-07T18:25:22Z
https://github.com/huggingface/datasets/pull/2874#issuecomment-914525294
https://api.github.com/repos/huggingface/datasets/issues/2874
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This PR extends the support in streaming mode for datasets that use `pathlib.Path`.

Related to: #2866.
CC: @severo 