In issue13597, Philip Jenvey points out:

“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”

Recent changes to the IO stack should allow stderr to be opened in fully unbuffered mode (and open(..., 'w', buffering=0) can be allowed too). Or at least it could be always line-buffered, even when redirected to a file.