Require isort on all Python files. (#8420)

tests/python-gpu/test_gpu_updaters.py

@@ -4,13 +4,10 @@ from typing import Any, Dict
 import numpy as np
 import pytest
 from hypothesis import assume, given, note, settings, strategies
+from xgboost.testing.params import cat_parameter_strategy, hist_parameter_strategy
 
 import xgboost as xgb
 from xgboost import testing as tm
-from xgboost.testing.params import (
-    hist_parameter_strategy,
-    cat_parameter_strategy,
-)
 
 sys.path.append("tests/python")
 import test_updaters as test_up