TST: Fix ignore text

Author: pllim

setup.cfg

@@ -11,7 +11,7 @@ filterwarnings =
     ignore:numpy.ufunc size changed:RuntimeWarning
     ignore:numpy.ndarray size changed:RuntimeWarning
     ignore::DeprecationWarning:ginga
-    ignore:is empty:UserWarning
+    ignore:Marker set named:UserWarning
 
 [flake8]
 # E501: line too long