title: builtins.TypeError TypeError: {2, 5, 23} is not JSON serializable
body: <p>This is my code.<br>
  I am getting a <code>JSON</code> not serializable error in <code>b.start_time</code>.</p>
  
  <pre><code>def rooms():
      rooms = Room.query.all()       
      formatted_rooms = []
      for r in rooms:
          book = Booking.query.filter_by(room_id=r.id).all()
          formatted_rooms.append( {           
                  'name' : r.name,
                  'floor': r.floor, 'location': r.location, 
                  'city' : r.city, 'capacity' : r.capacity,
                  'credits' : r.credits, 'amenities' : r.amenities ,
                  'room_id' : r.id,
                   'booking_details': { (b.start_time) 
                                       for b in book }
                                  } )
      return json.dumps(formatted_rooms) , 200, {'Content-Type':'application/json'}
  </code></pre>