string_join(): Some cleaning up of reference counting. In the

seqlen==1 clause, before returning item, we need to DECREF seq.  In
the res=PyString... failure clause, we need to goto finally to also
decref seq (and the DECREF of res in finally is changed to a
XDECREF).  Also, we need to DECREF seq just before the
PyUnicode_Join() return.