Require isort on all Python files. (#8420)

demo/gpu_acceleration/cover_type.py

@@ -1,7 +1,9 @@
-import xgboost as xgb
+import time
+
 from sklearn.datasets import fetch_covtype
 from sklearn.model_selection import train_test_split
-import time
+
+import xgboost as xgb
 
 # Fetch dataset using sklearn
 cov = fetch_covtype()