[3.13] gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849) (#141852)

gh-98552: flush std streams in the multiprocessing forkserver before fork (GH-141849)

* flush std streams in the multiprocessing forkserver before fork

* NEWS
(cherry picked from commit 58badb1711)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>

Lib/multiprocessing/forkserver.py

@@ -270,6 +270,7 @@ def sigchld_handler(*_unused):
                                     len(fds)))
                         child_r, child_w, *fds = fds
                         s.close()
+                        util._flush_std_streams()
                         pid = os.fork()
                         if pid == 0:
                             # Child