fix unicode literal in v4.test_convert

caused failure on Python 2 on Windows

because unicode!

IPython/nbformat/v4/tests/test_convert.py

@@ -35,7 +35,7 def test_upgrade_heading():
             v4m(source='#### foo bar multi-line'),
         ),
         (
-            v3h(source='ünìcö∂e–cønvërsioñ', level=4),
+            v3h(source=u'ünìcö∂e–cønvërsioñ', level=4),
             v4m(source=u'#### ünìcö∂e–cønvërsioñ'),
         ),
     ]: