Update repo to isort v5 (#6686)

* Run new isort

* More changes

* Update CI, CONTRIBUTING and benchmarks

tests/test_modeling_transfo_xl.py

@@ -25,7 +25,8 @@ from .test_modeling_common import ModelTesterMixin, ids_tensor
 
 if is_torch_available():
     import torch
-    from transformers import TransfoXLConfig, TransfoXLModel, TransfoXLLMHeadModel
+
+    from transformers import TransfoXLConfig, TransfoXLLMHeadModel, TransfoXLModel
     from transformers.modeling_transfo_xl import TRANSFO_XL_PRETRAINED_MODEL_ARCHIVE_LIST