bpo-31804: Fix multiprocessing.Process with broken standard streams (GH-6079) (GH-6080)

In some conditions the standard streams will be None or closed in the child process (for example if using "pythonw" instead of "python" on Windows).  Avoid failing with a non-0 exit code in those conditions.

Report and initial patch by poxthegreat.
(cherry picked from commit e756f66c83)

Co-authored-by: Antoine Pitrou <pitrou@free.fr>

Lib/multiprocessing/util.py

@@ -391,6 +391,20 @@ def _close_stdin():
     except (OSError, ValueError):
         pass
 
+#
+# Flush standard streams, if any
+#
+
+def _flush_std_streams():
+    try:
+        sys.stdout.flush()
+    except (AttributeError, ValueError):
+        pass
+    try:
+        sys.stderr.flush()
+    except (AttributeError, ValueError):
+        pass
+
 #
 # Start a program with only specified fds kept open
 #