Fix test, add highlight_source to ConverterPy.

This is a bit odd and inelegant but short of a
"supports_highlight_source" class-level attribute and logic to handle it
in the main() entrypoint, I'm not sure how else to resolve it.

converters/python.py

@@ -44,8 +44,13 b' class ConverterPy(Converter):'
     """
     extension = 'py'
 
-    def __init__(self, infile, show_prompts=True, show_output=True):
-        super(ConverterPy, self).__init__(infile)
+    def __init__(self, infile, highlight_source=False, show_prompts=True,
+                 show_output=True):
+        # Note that highlight_source is meaningless here, so even if it
+        # is passed as True we ignore it and propagate False to the
+        # superclass.
+        super(ConverterPy, self).__init__(infile,
+                                          highlight_source=False)
         self.show_prompts = show_prompts
         self.show_output = show_output