Update URL of tatoeba subset of xtreme
<s>To be more precise: `os.path.join` is replaced on-the-fly by `xjoin` anyway with patching, to extend it to remote files</s> Oh actually just ignore what I said: they were used to concatenate URLs, which is not recommended. Let me fix that again by appending using `+`
Updates the URL of the tatoeba subset of xtreme. Additionally, replaces `os.path.join` with `xjoin` to correctly join the URL segments on Windows.

Fix #3320 