#9559: Append data to single-file mailbox files if messages are only added

If messages were only added, a new file is no longer created and
renamed over the old file when flush() is called on an mbox, MMDF or
Babyl mailbox.

diff --cc Lib/mailbox.py
Simple merge

diff --cc Lib/test/test_mailbox.py
Simple merge

diff --cc Misc/NEWS
index 20a24071d8bf2bf32ccea90ffe29a47e97cc7c51,a8c274965d9934f0ca19687d5f31145226f0c1cc..4dcbfc2efca9bfcd4db628c563f3a3dd2e0a46f1
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -2,40 -2,10 +2,44 @@@
  Python News
  +++++++++++
  
 -What's New in Python 3.2.4
 -==========================
 +What's New in Python 3.3.0 Beta 2?
 +==================================
 +
 +*Release date: xx-xxx-2012*
 +
 +Core and Builtins
 +-----------------
 +
 +
 +Library
 +-------
 +
++- Issue #9559: If messages were only added, a new file is no longer
++  created and renamed over the old file when flush() is called on an
++  mbox, MMDF or Babyl mailbox.
++
 +- Issue 10924: Fixed mksalt() to use a RNG that is suitable for cryptographic
 +  purpose.
 +
 +Extension Modules
 +-----------------
 +
 +- Issue #15194: Update libffi to the 3.0.11 release.
 +
 +Tools/Demos
 +-----------
 +
 +- Issue #12605: The gdb hooks for debugging CPython (within Tools/gdb) have
 +  been enhanced to show information on more C frames relevant to CPython within
 +  the "py-bt" and "py-bt-full" commands:
 +    * C frames that are waiting on the GIL
 +    * C frames that are garbage-collecting
 +    * C frames that are due to the invocation of a PyCFunction
 +
 +What's New in Python 3.3.0 Beta 1?
 +==================================
  
 -*Release date: XX-XX-XXXX*
 +*Release date: 27-Jun-2012*
  
  Core and Builtins
  -----------------