Decrement references to python objects passed to Py_BuildValue

Decrement references to python objects passed to Py_BuildValue

Py_BuildValue when processing format 'O' will
  'Pass a Python object untouched (except for its reference count,
   which is incremented by one'

Basically this means if you are using a new reference to a PyObject
to pass to BuildValue (to be used with the 'O' format) the reference
*isn't* stolen so you really do need to DECREF it in order to ensure
it gets cleaned up.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>