Update: WMT - use mirror links
So much better - thank you for doing that, @lhoestq! Also fixed the `uncorpus` urls for wmt19 ru-en and zh-en for https://github.com/huggingface/datasets/issues/1893 Thanks!
Can this be merged sooner? 
I manually update it and it works well.
As asked in #1892 I created mirrors of the data hosted on statmt.org and updated the wmt scripts.
Now downloading the wmt datasets is blazing fast :)

cc @stas00 @patrickvonplaten 