Thank you Dennis.  I was just about to post that after installing 3.8.3rc1 and 3.9.0a6 in a Macbook Air, with identical idlelib code, I confirmed the problem as specific to 3.9 and that

$ python3.9 -X oldparser -m idlelib

works around the issue.  My python development desktop is currently in a  repair shop. I will try to verify the fix on Windows master when I can.

idlelib.pyshell.ModifiedInterpreter subclasses code.InteractiveInterpreter.  After each 'Enter', something calls compile(current_lines,  '<input>','single').  Something changed that makes a difference in one of the files.

The only thing I found is that the error for 'if 1:\n' changed from "SyntaxError: unexpected EOF ..." to "IndentationError: expected ...".  Since issubclass(IndentationError, SyntaxError), I would not expect that to be a problem, but if Pablo fixed this, good enough for me.  So I am closing until someone says otherwise.