Require isort on all Python files. (#8420)

demo/rank/rank_sklearn.py

@@ -1,7 +1,8 @@
 #!/usr/bin/python
-import xgboost as xgb
 from sklearn.datasets import load_svmlight_file
 
+import xgboost as xgb
+
 #  This script demonstrate how to do ranking with XGBRanker
 x_train, y_train = load_svmlight_file("mq2008.train")
 x_valid, y_valid = load_svmlight_file("mq2008.vali")