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

gh-135335: flush stdout/stderr in forkserver after preloading modules (#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.

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