Fix Python 3.5 compat

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/1005

Differential Revision: D16751489

Pulled By: myleott

fbshipit-source-id: 6e372ac23643e32a3791044c13f4466bdc28f049

fairseq/tasks/sentence_ranking.py

@@ -144,7 +144,7 @@ class SentenceRankingTask(FairseqTask):
                 }
             )
 
-        label_path = f"{get_path('label', split)}.label"
+        label_path = '{}.label'.format(get_path('label', split))
         if os.path.exists(label_path):
             dataset.update(
                 target=RawLabelDataset([