Convert label to str in graph_transitions.py

If the label of a state is not a string, trying to add it to the graphviz Graph will fail. This could be the case when using `FSMFieldMixin` instead of directly using `FSMField`.

django_fsm/management/commands/graph_transitions.py

@@ -105,9 +105,9 @@ def generate_dot(fields_data):
 
         final_states = targets - sources
         for name, label in final_states:
-            subgraph.node(name, label=label, shape="doublecircle")
+            subgraph.node(name, label=str(label), shape="doublecircle")
         for name, label in (sources | targets) - final_states:
-            subgraph.node(name, label=label, shape="circle")
+            subgraph.node(name, label=str(label), shape="circle")
             if field.default:  # Adding initial state notation
                 if label == field.default:
                     initial_name = node_name(field, "_initial")