The following command should not produce any output but it does:

  $ ./python >/dev/null -c 'import subprocess as S, sys; S.call([sys.executable, "-c", "import sys; print(42, file=sys.stderr)"], stderr=S.STDOUT)'

Its stdout is redirected to /dev/null. It starts a subprocess with its
stderr redirected to stdout. See "Redirect subprocess stderr to
stdout" [1] on Stackoverflow.

[1] http://stackoverflow.com/questions/11495783/redirect-subprocess-stderr-to-stdout

I've uploaded a patch that fixes the issue on POSIX.

Please, run the provided test (in the patch), to see whether the code
should be fixed on Windows too (it might work as is there).

No documentation changes are required.

Please, review.