Require isort on all Python files. (#8420)

demo/aft_survival/aft_survival_viz_demo.py

@@ -6,9 +6,10 @@ This demo uses 1D toy data and visualizes how XGBoost fits a tree ensemble. The
 model starts out as a flat line and evolves into a step function in order to account for
 all ranged labels.
 """
-import numpy as np
-import xgboost as xgb
 import matplotlib.pyplot as plt
+import numpy as np
+
+import xgboost as xgb
 
 plt.rcParams.update({'font.size': 13})