> I'm surprised to hear that stderr is line buffered by default.
> Historically stderr is never buffered (at least on POSIX) and for good
> reason: errors should be seen immediately
> 
> Was this an oversight in migrating stdin/out/err to the new io module?

Until recently it wasn't possible to have an unbuffered text stream with
the new io stack. Now it's possible in write-only mode, although the
open() builtin hasn't been updated for it.