Switch to pandas reader for text dataset
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.