Update repo to isort v5 (#6686)

* Run new isort

* More changes

* Update CI, CONTRIBUTING and benchmarks

tests/test_benchmark_tf.py

@@ -9,6 +9,7 @@ from transformers.testing_utils import require_tf
 
 if is_tf_available():
     import tensorflow as tf
+
     from transformers import TensorFlowBenchmark, TensorFlowBenchmarkArguments