fix deprecated np.matrix in test_sprm.py

Replace np.matrix with np.asarray for NumPy compatibility.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: SvenSerneels

src/direpack/test/test_sprm.py

@@ -26,7 +26,7 @@ def tearDownClass(cls):
 
     def setUp(self):
         self.data = ps.read_csv("./data/Returns_shares.csv")
-        self.datav = np.matrix(self.data.values[:, 2:8].astype("float64"))
+        self.datav = np.asarray(self.data.values[:, 2:8].astype("float64"))
         self.x = self.datav[:, 0:5]
         self.y = self.datav[:, 5]
         self.n = self.data.shape[0]