Fixes issue #15756: subprocess.poll() now properly handles errno.ECHILD to

return a returncode of 0 when the child has already exited or cannot be
waited on.

diff --cc Misc/NEWS
index 248b79ccc3f773f68a1305af49ca6a8181e8ed4b,245cf1a677e08ceea6e329d971995a3320be4e28..0012f1ea66957e877c8d01e3a12c67efc4fad886
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -15,9 -13,10 +15,13 @@@ Core and Builtin
  Library
  -------
  
+ - Issue #15756: subprocess.poll() now properly handles errno.ECHILD to
+   return a returncode of 0 when the child has already exited or cannot
+   be waited on.
+ 
 +Extension Modules
 +-----------------
 +
  
  What's New in Python 3.3.0?
  ===========================