python: fix output of twa_run

* src/twaalgos/emptiness.hh, src/twaalgos/emptiness.cc: Declare the
operator<< for twa_run, not for twa_run_ptr (the shared_ptr
automatically forward operator<<).
* wrap/python/spot_impl.i: Add __str__ to twa_run, not twa_run_ptr.

wrap/python/spot_impl.i

@@ -383,7 +383,7 @@ namespace std {
   }
 }
 
-%extend spot::twa_run_ptr {
+%extend spot::twa_run {
   std::string __str__()
   {
     std::ostringstream os;