please update example/server.py

[kkdd]

Hello,
Could you please update example/server.py for the latest python environment?
Otherwise, the following error occurred:

$ python3 example/server.py 
Traceback (most recent call last):
  File "example/server.py", line 29, in <module>
    class WebSocketHandler(tornado.websocket.WebSocketHandler):
  File "example/server.py", line 35, in WebSocketHandler
    @tornado.gen.engine
AttributeError: module 'tornado.gen' has no attribute 'engine'
$
$ pip3 list | grep -e appdirs -e packaging -e pyparsing -e six -e tornado
appdirs             1.4.3
packaging           19.2 
pyparsing           2.4.4 
six                 1.13.0 
tornado             6.0.3