python: allow iterating over the successors of a state

Fixes #118.

* spot/twa/twagraph.hh: Avoid using graph_t::state to help Swig.
* wrap/python/spot_impl.i: Add a __str__ function for acc_cond::mark_t.
* doc/org/tut21.org: Add the Python version.
* doc/org/tut.org: Move tut21.org to the Python/C++ section.
* NEWS: Update.

wrap/python/spot_impl.i

@@ -476,6 +476,15 @@ namespace std {
   }
 }
 
+%extend spot::acc_cond::mark_t {
+  std::string __str__()
+  {
+    std::ostringstream os;
+    os << *self;
+    return os.str();
+  }
+}
+
 %extend spot::twa_run {
   std::string __str__()
   {