Terry, have you considered also doing a top level idlelib -> _idlelib rename in addition to the file level name changes?

My rationale for suggesting that:

1. The idle2 vs idle3 distinction will be clear at a glance (as they'll be in different directories)
2. The package level rename handles bifurcation of files that aren't themselves being renamed
3. You can subsequently iterate on the internal APIs without worrying about third party breakage
4. You set the stage for eventually moving IDLE to being an independently versioned implicitly pip-installed component (perhaps with an "ensure_default_ide" module akin to "ensurepip")

In relation to that last point, while I think it's important for CPython to come with a default IDE, I *don't* think it's important for the Python standard library to provide a default IDE-building toolkit (and, indeed, Linux distros tend to separate out not only IDLE, but also Tcl/Tk support in general, from the default Python installation).