Fix Python 3 issue and bug

- RuntimeError: dictionary changed size during iteration

- `id()` doesn't exist

people_velocity_tracker/scripts/tracker.py

@@ -123,9 +123,9 @@ def spin(self):
         while not rospy.is_shutdown():
             # Remove People Older Than timeout param
             now = rospy.Time.now()
-            for p in self.people.values():
+            for p in list(self.people.values()):
                 if now - p.age() > self.TIMEOUT:
-                    del self.people[p.id()]
+                    del self.people[p.get_id()]
             self.publish()
             rate.sleep()
 
@@ -134,7 +134,7 @@ def publish(self):
         pl = People()
         pl.header.frame_id = None
 
-        for p in self.people.values():
+        for p in list(self.people.values()):
             p.publish_markers(self.mpub)
             frame, person = p.get_person()
             pl.header.frame_id = frame