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The interning of short strings violates the refcnt==1 
assumption for _PyString_Resize.

A simple fix is to boost the initial value of "totalnew" by 1. 
Combined with an NULL argument to 
PyString_FromStringAndSize, this assures that resulting 
format string is not interned.  This will remain true even if 
the implementation of PyString_FromStringAndSize changes 
because only the uninitialized strings that can be interned 
are those of zero length.

Fixed and added a test case.  See:
  Modules/datetimemodule.c 1.67
  Lib/test/test_datetime.py 1.45