Why does High Sierra complain about IDLE? I've installed an updated version of python 2.7, which comes with the IDLE development environment. 

High Sierra is giving me warnings that it needs to be updated (which I took to mean that it's 32-bit, but it's not.)

I can't remember the exact warning, but it's the 'this app is not optimized for MacOS, consult the developer' thing.
That specific error message means that the app (or a library, or launcher or component of some sort) is a 32 bit app and not a 64 bit app. Something in the Python installation is triggering that message even if python itself is 64bit.

And in an upcoming release of macOS 32bit apps will no longer work. Check python.org for FAQs/Discussions/Support/etc. for when it will be compiled as a 64bit app.