Pin tornado to 6.1 in CI

[texodus]

Perspective/Jupyterlab tests dump random errors coincident with the release of Tornado 6.2 today. I don't yet know if this is an existing bug in Perspective that has been exposed, or in JupyterLab or Tornado, so I have not modified the setup.py bounds.

I'll be looking into removing the manual dependency installation from azure as well, as IMO it is unacceptable to have CI break when dependencies change.

Exception in callback <TaskWakeupMethWrapper object at 0x10dd9f1c0>(<Future finis... GMT\r\n\r\n'>)
handle: <Handle <TaskWakeupMethWrapper object at 0x10dd9f1c0>(<Future finis... GMT\r\n\r\n'>)>
Traceback (most recent call last):
  File "/usr/local/Cellar/python@3.9/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
RuntimeError: Cannot enter into task <Task pending name='Task-13' coro=<HTTP1ServerConnection._server_request_loop() running at /usr/local/lib/python3.9/site-packages/tornado/http1connection.py:825> wait_for=<Future finished result=b'GET /static...7 GMT\r\n\r\n'> cb=[IOLoop.add_future.<locals>.<lambda>() at /usr/local/lib/python3.9/site-packages/tornado/ioloop.py:687]> while another task <Task pending name='Task-224' coro=<PeriodicCallback._run() running at /usr/local/lib/python3.9/site-packages/tornado/ioloop.py:921> cb=[IOLoop.add_future.<lo