fix failing deprecated test

converters/base.py

@@ -143,7 +143,7 b' class Converter(Configurable):'
     raw_as_verbatim = False
 
 
-    def __init__(self, infile=None, config=None, exclude=[], **kw):
+    def __init__(self, infile='', config=None, exclude=[], **kw):
         super(Converter,self).__init__(config=config)
 
         #DocStringInheritor.__init__(self=config)

tests/test_simple.py

@@ -1,4 +1,4 b''
-from nbconvert import main, converters , NbconvertApp
+from nbconvert import converters, NbconvertApp
 from converters.rst import ConverterRST
 import nose.tools as nt