OK, I got it. 

In my opinion it would nice if user can either convert file to utf8 or
to do nothing and add new encodings declaration or cancel. Current
"Cancel" gives an Decoding error. If you give an encodings that doesn't
exist, it shouldn't  destroy IDLE. Hoping its not my mistake, cause I do
not have all files from 3.1a  - just those from idlelib.