procutil: explain better why line buffering is not possible The sentence “On Python 3, buffered binary streams can't be set line-buffered.” was imprecise, as all streams are just Python classes and we can implement our own (which we did).

tests: make subprocess handling reusable for different tests in test-stdio.py

procutil: back out 8403cc54bc83 (make ....procutil.stderr unbuffered) Changeset 8403cc54bc83 introduced code that opens a second file object referring to the stderr file descriptor. This broke tests on Windows. The reason is that on Windows, sys.stderr is buffered and procutil.stderr closed the file descriptor when it got garbage collected before sys.stderr had the chance to flush buffered data. `procutil.stdout` had the same problem for a long time, but we didn’t realize, as in CI test runs, stdout is not a TTY and in this case no second file object is opened.