Change some uses of cStringIO.StringIO to io.StringIO.

This is undoubtedly insufficient and in some cases just as broken as before.

Lib/mhlib.py

@@ -697,10 +697,7 @@ class Message(mimetools.Message):
         encoding = self.getencoding()
         if not decode or encoding in ('', '7bit', '8bit', 'binary'):
             return self.fp.read()
-        try:
-            from cStringIO import StringIO
-        except ImportError:
-            from StringIO import StringIO
+        from io import StringIO
         output = StringIO()
         mimetools.decode(self.fp, output, encoding)
         return output.getvalue()