[3.6] bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (GH-4073). (#4075)

* bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (#4073)

(cherry picked from commit daeefd2e04)

* [3.6] bpo-28326: Fix multiprocessing.Process when stdout and/or stderr is closed or None. (GH-4073).
(cherry picked from commit daeefd2e04)

Lib/multiprocessing/popen_fork.py

@@ -14,8 +14,14 @@ class Popen(object):
     method = 'fork'
 
     def __init__(self, process_obj):
-        sys.stdout.flush()
-        sys.stderr.flush()
+        try:
+            sys.stdout.flush()
+        except (AttributeError, ValueError):
+            pass
+        try:
+            sys.stderr.flush()
+        except (AttributeError, ValueError):
+            pass
         self.returncode = None
         self._launch(process_obj)