fix: remove type checker exception by adding an assert statement

src/snanosm/mealy.py

@@ -112,7 +112,7 @@ class Machine(Generic[Input]):
         # dict[hash_origin, dict[hash_transition_input, Transition]]
         if self.__current_state_hash not in self.__transitions.keys():
             raise ValueError("Processing input for a state with no transitions.")
-        # noinspection PyTypeChecker
+        assert self.__current_state_hash is not None
         transitions = self.__transitions[self.__current_state_hash]
         hi = hash(i)
         transition: Optional[Transition] = None