mark_t: sets() no longer returns a vector

spot::mark_t::sets() was modified so that it now returns an iterable
object rather than an std::vector<unsigned>.

* NEWS: Mention the modification.
* python/spot/impl.i: Declares mark_container as iterable to SWIG.
* spot/parseaut/parseaut.yy: Adapts to the modification.
* spot/twa/acc.hh: Implement the modification.
* tests/python/acc_cond.ipynb: Adapts to the modification.

python/spot/impl.i

@@ -787,6 +787,14 @@ def state_is_accepting(self, src) -> "bool":
    }
 }
 
+%extend spot::internal::mark_container {
+  swig::SwigPyIterator* __iter__(PyObject **PYTHON_SELF)
+   {
+      return swig::make_forward_iterator_np(self->begin(), self->begin(),
+				         self->end(), *PYTHON_SELF);
+   }
+}
+
 %extend spot::twa_graph {
   unsigned new_univ_edge(unsigned src, const std::vector<unsigned>& v,
                          bdd cond, acc_cond::mark_t acc = 0U)