Fix flake8 code E226 by editing test_Data formatting

Author: sadielbartholomew

cf/test/test_Data.py

@@ -3732,7 +3732,7 @@ def test_Data_var(self):
             e = np.ma.array(e.array)
 
             self.assertTrue((e.mask == b.mask).all())
-            self.assertTrue(np.allclose(e, b), f"e={e}\nb={b}\ne-b={e-b}")
+            self.assertTrue(np.allclose(e, b), f"e={e}\nb={b}\ne-b={e - b}")
 
         #  Weighted ddof = 1
         for axis in axis_combinations(a):