For the stdlib in general, What's New in x.y.0 is sufficient because there is not supposed to be anything new in bugfix releases except for bugfixes.  2.7 has a security enhancement exemption, and a corresponding section in the 2.7 What's New for security enhancements in maintenance releases.

A similar section for Idle could be added to What's New for each Python version.  But thinking about it, I would rather have a separate document listed in the Idle help menu and advertised in the Idle sign-on message. In particular, I would replace the useless "Type "copyright", "credits" or "license()" for more information." with "See HELP/WHAT'S NEW for information about changes in each release."