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/popen_fork.py

@@ -14,14 +14,7 @@ class Popen(object):
     method = 'fork'
 
     def __init__(self, process_obj):
-        try:
-            sys.stdout.flush()
-        except (AttributeError, ValueError):
-            pass
-        try:
-            sys.stderr.flush()
-        except (AttributeError, ValueError):
-            pass
+        util._flush_std_streams()
         self.returncode = None
         self.finalizer = None
         self._launch(process_obj)