Update repo to isort v5 (#6686)

* Run new isort

* More changes

* Update CI, CONTRIBUTING and benchmarks

tests/test_modeling_longformer.py

@@ -25,14 +25,15 @@ from .test_modeling_common import ModelTesterMixin, ids_tensor, random_attention
 
 if is_torch_available():
     import torch
+
     from transformers import (
         LongformerConfig,
-        LongformerModel,
         LongformerForMaskedLM,
+        LongformerForMultipleChoice,
+        LongformerForQuestionAnswering,
         LongformerForSequenceClassification,
         LongformerForTokenClassification,
-        LongformerForQuestionAnswering,
-        LongformerForMultipleChoice,
+        LongformerModel,
         LongformerSelfAttention,
     )