A suggestion: rather than trying to maintain a separate IDLE news file ("don't repeat yourself), it might be much easier to link to the IDLE section in each releases's formatted CHANGELOG now included in Python 3 docsets.  For example:

https://docs.python.org/3/whatsnew/changelog.html#idle
or
https://docs.python.org/3.8/whatsnew/changelog.html#idle

Yes, users would have to scroll down to each release's IDLE section but I suspect that wouldn't be a huge burden considering how infrequently change files are consulted.  To be even more useful, IDLE could first attempt to find the changelog in the Python installer's bundled copy of the docset like the F1 key does.  For example, with the macOS installer, the corresponding file URL is:

file:///Library/Frameworks/Python.framework/Versions/3.8/Resources/English.lproj/Documentation/whatsnew/changelog.html#idle