Improve UniformGrid assert fail test

The way the test message was compared brought problems with
different python versions.

src/grid/tests/test_cubic.py

@@ -391,9 +391,10 @@ def test_raises_error_correctly_when_constructing_uniform_grid(self):
         )
         # Test shape is always positive
         with self.assertRaises(ValueError) as err:
-            UniformGrid(proper_origin, proper_axes, np.array([5, -1, 2]))
+            grid_shape = np.array([5, -1, 2])
+            UniformGrid(proper_origin, proper_axes, grid_shape)
         self.assertEqual(
-            "Number of points in each direction should be positive, got shape=[5, -1, 2]",
+            f"Number of points in each direction should be positive, got shape={list(grid_shape)}",
             str(err.exception),
         )
         # Test the weights are correct