Update Dask and Distributed minimum version to 2.12 for Python 3.8 compatibility

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Win 10
• Modin installed from (source or binary): pip
• Modin version: 0.6.3
• Python version: 3.8.0
• Exact command to reproduce: import modin.pandas as pd

Describe the problem

Fresh install of modin, failure to import. Have tried pip --force-reinstall most dependencies.

Source code / logs

UserWarning: The Dask Engine for Modin is experimental.
Traceback (most recent call last):
  File "C:\WERK\backtestStrat3\backtest.py", line 2, in <module>
    import modin.pandas as pd
  File "C:\Python\lib\site-packages\modin\pandas\__init__.py", line 226, in <module>
    client = Client()
  File "C:\Python\lib\site-packages\distributed\client.py", line 727, in __init__
    self.start(timeout=timeout)
  File "C:\Python\lib\site-packages\distributed\client.py", line 892, in start
    sync(self.loop, self._start, **kwargs)
  File "C:\Python\lib\site-packages\distributed\utils.py", line 334, in sync
    raise exc.with_traceback(tb)
  File "C:\Python\lib\site-packages\distributed\utils.py", line 318, in f
    result[0] = yield future
  File "C:\Python\lib\site-packages\tornado\gen.py", line 735, in run
    value = future.result()
  File "C:\Python\lib\site-packages\distributed\client.py", line 955, in _start
    self.cluster = await LocalCluster(
  File "C:\Python\lib\site-packages\distributed\deploy\spec.py", line 364, in _
    await self._start()
  File "C:\Python\lib\site-packages\distributed\deploy\spec.py", line 284, in _start
    self.scheduler = await self.scheduler
  File "C:\Python\lib\site-packages\distributed\scheduler.py", line 1219, in start
    await self.listen(addr, listen_args=self.listen_args)
  File "C:\Python\lib\site-packages\distributed\core.py", line 321, in listen
    await listener.start()
  File "C:\Python\lib\site-packages\distributed\comm\tcp.py", line 432, in start
    self.tcp_server.add_sockets(sockets)
  File "C:\Python\lib\site-packages\tornado\tcpserver.py", line 165, in add_sockets
    self._handlers[sock.fileno()] = add_accept_handler(
  File "C:\Python\lib\site-packages\tornado\netutil.py", line 279, in add_accept_handler
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)
  File "C:\Python\lib\site-packages\tornado\platform\asyncio.py", line 99, in add_handler
    self.asyncio_loop.add_reader(fd, self._handle_events, fd, IOLoop.READ)
  File "C:\Python\lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError
NotImplementedError
Error in atexit._run_exitfuncs:
Traceback (most recent call last):
  File "C:\Python\lib\contextlib.py", line 131, in __exit__
    self.gen.throw(type, value, traceback)
  File "C:\Python\lib\site-packages\distributed\utils.py", line 186, in ignoring
    yield
  File "C:\Python\lib\site-packages\distributed\deploy\spec.py", line 607, in close_clusters
    cluster.close(timeout=10)
  File "C:\Python\lib\site-packages\distributed\deploy\cluster.py", line 83, in close
    return self.sync(self._close, callback_timeout=timeout)
  File "C:\Python\lib\site-packages\distributed\deploy\cluster.py", line 162, in sync
    return sync(self.loop, func, *args, **kwargs)
  File "C:\Python\lib\site-packages\distributed\utils.py", line 334, in sync
    raise exc.with_traceback(tb)
  File "C:\Python\lib\site-packages\distributed\utils.py", line 318, in f
    result[0] = yield future
  File "C:\Python\lib\site-packages\tornado\gen.py", line 735, in run
    value = future.result()
  File "C:\Python\lib\site-packages\distributed\deploy\spec.py", line 380, in _close
    self.scale(0)
  File "C:\Python\lib\site-packages\distributed\deploy\spec.py", line 444, in scale
    v["name"] for v in self.scheduler_info["workers"].values()
KeyError: 'workers'

[PMeira]

  File "C:\Python\lib\site-packages\tornado\platform\asyncio.py", line 99, in add_handler
    self.asyncio_loop.add_reader(fd, self._handle_events, fd, IOLoop.READ)
  File "C:\Python\lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError

That’s a Tornado + Python 3.8 issue on Windows. Python 3.8 changed some defaults. I posted a comment earlier today, Dask Distributed probably needs a small patch: https://github.com/dask/distributed/pull/3249#issuecomment-569539093

[devin-petersohn]

@itamarst It is probably worth updating our minimum Dask and Distributed versions to ensure compatibility with Python 3.8.