Fix memory leak in low-level module.

The census-event string was not decrefed and we therefore leaked memory.

_msprimemodule.c

@@ -2861,6 +2861,7 @@ Simulator_parse_demographic_events(Simulator *self, PyObject *py_events)
     Py_XDECREF(instantaneous_bottleneck_s);
     Py_XDECREF(initial_size_s);
     Py_XDECREF(growth_rate_s);
+    Py_XDECREF(census_event_s);
     return ret;
 }