Add "r" literal to docstring

The docstring contains a backslash.

src/cdiff.py

         self._hunk_list.append((attr, line))
 
     def mdiff(self):
-        """The difflib._mdiff() function returns an interator which returns a
+        r"""The difflib._mdiff() function returns an interator which returns a
         tuple: (from line tuple, to line tuple, boolean flag)
 
         from/to line tuple -- (line num, line text)