macros.  The 'op' argument is then the result from PyObject_MALLOC,
and that can of course be NULL.  In that case, PyObject_Init[Var]
would raise a SystemError with "NULL object passed to
PyObject_Init[Var]".  But there's nothing the caller of the macro can
do about this.  So PyObject_Init[Var] should call just PyErr_NoMemory.

Will backport.