Pytorchltr

* Fixes compatibility for python 3.5

This release:
* Improves performance when loading SVMRank datasets
* Adds Istella and Istella-X datasets

First official release of PyTorchLTR! 🎉

Core functionality:
* Loading SVMRank datasets.
* Automatically download and process several public LTR datasets.
* Commonly used pairwise loss functions.
* Basic evaluation metrics and integration with pytrec_eval.