Update repo to isort v5 (#6686)

* Run new isort

* More changes

* Update CI, CONTRIBUTING and benchmarks

tests/test_optimization_tf.py

@@ -8,7 +8,8 @@ if is_tf_available():
     import tensorflow as tf
     from tensorflow.python.eager import context
     from tensorflow.python.framework import ops
-    from transformers import create_optimizer, GradientAccumulator
+
+    from transformers import GradientAccumulator, create_optimizer
 
 
 @require_tf