Specify src path for isort. (#8867)

tests/python/test_quantile_dmatrix.py

@@ -4,6 +4,8 @@ import numpy as np
 import pytest
 from hypothesis import given, settings, strategies
 from scipy import sparse
+
+import xgboost as xgb
 from xgboost.testing import (
     IteratorForTest,
     make_batches,
@@ -15,8 +17,6 @@ from xgboost.testing import (
 )
 from xgboost.testing.data import np_dtypes
 
-import xgboost as xgb
-
 
 class TestQuantileDMatrix:
     def test_basic(self) -> None: