Fix examples/translation/t5 to use newstest2014 rather than newstest2013
Resolves #3759, in addition to minor nits: fixed a bug with argparse arguments + more pythonic file handling + formatted with black and isort. 

Please note that I have not yet run the evaluation script against the full newstest2014 test set, as it is rather compute intensive, so the disclaimer at the top of the README.md about the score gap between the pre-trained and fine-tuned models is only ostensibly accurate to the score gap on newstest2013, not newstest2014.
Hey @tholiao, 

Thanks a lot for the PR :-) This looks good so far. Don't worry about running the script against the evaluation set - we can do this once this is merged! 

Can you make sure that the `run_examples_torch` pass? Don't worry too much about the `check_code_quality` test - there have been some issues with `isort` and I can manually fix that later. <|||||>Should be fine now. <|||||>Hi @tholiao, I went into your PR branch and checked the `make style` issues. It seems like you have other params set up for `black` than this lib. Also `isort` seems to have some problems with the imports here. 

I added the tiny changes, I suggested above and correctly formatted everything (black and isort) in this PR https://github.com/huggingface/transformers/pull/3817. The PR uses your commits, so you are an author of the commit :-) 