[3.13] gh-135335: flush stdout/stderr in forkserver after preloading modules (GH-135338) (#135671)

gh-135335: flush stdout/stderr in forkserver after preloading modules (GH-135338)

If a preloaded module writes to stdout or stderr, and the stream is buffered,
child processes will inherit the buffered data after forking. Attempt to
prevent this by flushing the streams after preload.
(cherry picked from commit 9877d191f441741fc27ae5e7a6dd7ab6d4bcc6b7)

Co-authored-by: Duane Griffin <duaneg@dghda.com>
Co-authored-by: Mikhail Efimov <efimov.mikhail@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>