Require isort on all Python files. (#8420)

demo/dask/dask_callbacks.py

@@ -3,12 +3,12 @@ Example of using callbacks with Dask
 ====================================
 """
 import numpy as np
-import xgboost as xgb
-from xgboost.dask import DaskDMatrix
-from dask.distributed import Client
-from dask.distributed import LocalCluster
+from dask.distributed import Client, LocalCluster
 from dask_ml.datasets import make_regression
 from dask_ml.model_selection import train_test_split
+from xgboost.dask import DaskDMatrix
+
+import xgboost as xgb
 
 
 def probability_for_going_backward(epoch):