Plugin HttpObserver broken since 0.17 because of change in AsyncHTTPClient from Tornado 5

[sboisson]

The plugin HttpObserver cannot be loaded because of unexpected io_loop argument when creating an instance of tornado.httpclient.AsyncHTTPClient:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/lib/python3.8/site-packages/circus/plugins/__init__.py", line 260, in main
    plugin = factory(args.endpoint, args.pubsub,
  File "/lib/python3.8/site-packages/circus/plugins/http_observer.py", line 17, in __init__
    self.http_client = AsyncHTTPClient(io_loop=self.loop)
  File "/lib/python3.8/site-packages/tornado/httpclient.py", line 206, in __new__
    instance = super(AsyncHTTPClient, cls).__new__(cls, **kwargs)  # type: ignore
  File "/lib/python3.8/site-packages/tornado/util.py", line 288, in __new__
    instance.initialize(*args, **init_kwargs)
TypeError: initialize() got an unexpected keyword argument 'io_loop'

Circus v0.17 upgraded tornado to v5, but the io_loop argument (deprecated since version v4.1) has been removed.