Vasyl, this line in your code is causing the problem:

    for row in readed:

The error reasonably clear states that it's due to your `fieldnames' parameter being non-iterable. Again, this has nothing whatsoever to do with `dialect' argument and documentation of Sniffer. 

Please, do not open new issues about this. This is not a issue with csv library or its documentation. Try to use use stackoverflow or python-tutor list to get help on this.
https://mail.python.org/pipermail/tutor/