Run black over arrange tests

tests/test_arrange.py

@@ -94,6 +94,7 @@ def test_arrange_dock_bottom():
     assert widgets == {child, header}
     assert spacing == Spacing(0, 0, 1, 0)
 
+
 def test_arrange_dock_badly():
     child = Widget(id="child")
     child.styles.dock = "nowhere"