Switch to pandas reader for text dataset
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[]
[]
If the windows tests in the CI pass, today will be a happy day
Following the discussion in #622 , it appears that there's no appropriate ways to use the payrrow csv reader to read text files because of the separator.

In this PR I switched to pandas to read the file.

Moreover pandas allows to read the file by chunk, which means that you can build the arrow dataset from a text file that is bigger than RAM (we used to have to shard text files an mentioned in https://github.com/huggingface/datasets/issues/610#issuecomment-691672919)

From a test that I did locally on a 1GB text file, the pyarrow reader used to run in 150ms while the new one takes 650ms (multithreading off for pyarrow). This is probably due to chunking since I am having the same speed difference by calling `read()` and calling `read(chunksize)` + `readline()` to read the text file.
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/689/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/689', 'html_url': 'https://github.com/huggingface/datasets/pull/689', 'diff_url': 'https://github.com/huggingface/datasets/pull/689.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/689.patch', 'merged_at': '2020-09-30T16:45:31Z'}