flush std streams in the multiprocessing forkserver before fork

Author: gpshead

Lib/multiprocessing/forkserver.py

@@ -326,6 +326,7 @@ 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