Require isort on all Python files. (#8420)

demo/rmm_plugin/rmm_mgpu_with_dask.py

@@ -1,8 +1,9 @@
-import xgboost as xgb
-from sklearn.datasets import make_classification
 import dask
 from dask.distributed import Client
 from dask_cuda import LocalCUDACluster
+from sklearn.datasets import make_classification
+
+import xgboost as xgb
 
 
 def main(client):

demo/rmm_plugin/rmm_singlegpu.py

@@ -1,7 +1,8 @@
-import xgboost as xgb
 import rmm
 from sklearn.datasets import make_classification
 
+import xgboost as xgb
+
 # Initialize RMM pool allocator
 rmm.reinitialize(pool_allocator=True)
 # Optionally force XGBoost to use RMM for all GPU memory allocation, see ./README.md