In Python3.8 and prior, you can type this in a shell session
------------------------------------------------------------

Python 3.8.2 (v3.8.2:7b3ab5921f, Feb 24 2020, 17:52:18) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license()" for more information.
>>> def square(x):
	s = x ** 2
	return s


In Python3.9, the input is terminated prematurely
-------------------------------------------------

Python 3.9.0a6 (v3.9.0a6:bc1c8af8ef, Apr 27 2020, 17:05:28) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license()" for more information.
>>> def square(x):
	s = x ** 2             # <-- On first return, the statement evaluates
	
>>> 

>>> square(5)
25

git bisect says that this was fixed here:

commit b94dbd7ac34dc0c79512656eb17f6f07e09fca7a
Author: Pablo Galindo <Pablogsal@gmail.com>
Date:   Mon Apr 27 18:35:58 2020 +0100

    bpo-40334: Support PyPARSE_DONT_IMPLY_DEDENT in the new parser (GH-19736)

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.