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

[miss-islington]

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 9877d19)

Co-authored-by: Duane Griffin [email protected]
Co-authored-by: Mikhail Efimov [email protected]
Co-authored-by: Victor Stinner [email protected]

• Issue: Fork server doesn't flush stdout/stderr after preloading modules, potentially leaving buffered data to be inherited by child processes #135335