I like the idea, though it's really just nice to have. This is a very common in IDEs these days, and whoever finds it annoying will be able to disable it.

If we do this, we should go all the way and close square and curly brackets, parenthesis, and quotes (including triple quotes!).

Also, any implementation must allow typing the closing parenthesis to actually just move the cursor beyond the automatically generated closer. Getting this right is more difficult than it sounds! But certainly possible.

I actually seem to recall having seen this implemented for IDLE quite a few years ago. I might even have been the one to implement it... I can't see any mention in IDLE-Spoon, IdleX or VIDLE, though. I'll take a look through my old files when I get the chance back home.