Fix pathlib patches for streaming
Fix issue https://github.com/huggingface/datasets/issues/2866 (for good this time)

`counter` now works in both streaming and non-streaming mode.
And the `AttributeError: 'str' object has no attribute 'as_posix'` related to the patch of Path.open is fixed as well

Note : the patches should only affect the datasets module, not the user's ones ! That's why we should probably use something else than patch.object to patch the Path class' methods.

cc @severo @albertvillanova 