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.

NEWS

@@ -48,6 +48,9 @@ New in spot 2.3.3.dev (not yet released)
     synonym for spot::twa::prop_universal() to help backward
     compatibility.
 
+  - spot::acc_cond::mark_t::sets() now returns an internal iterable
+    object instead of an std::vector<unsigned>.
+
   Deprecation notice:
 
   - spot::dtwa_complement() used to work only on deterministic