make tick protected

pydyton/core.py

@@ -17,9 +17,9 @@ class TickProcess(Process):
         super().__init__()
         if tick < 1:
             raise ValueError('tick must be greatert than or equal to 1')
-        self.__tick = tick
+        self._tick = tick
 
     def tick(self):
-        return self.env.timeout(self.__tick)
+        return self.env.timeout(self._tick)