Getting 404 error from WS URL in RTMClient#start() with aiohttp 3.7.0 or lower versions

RTMClient.start() failing as of 2021-01-09. Connection attempted with both slack_sdk and (legacy) slackclient.

The Slack SDK version

slack-sdk 3.1.1

(Also occuring on slackclient 2.9.3)

Python runtime version

Python 3.6.7

OS info

Microsoft Windows [Version 10.0.18362.1256]

Steps to reproduce:

> import slack_sdk.rtm as rtm

> rtm_client = rtm.RTMClient(token=token) # also attempted with ssl_context provided
> rtm_client.start()

Testing via https://api.slack.com/methods/rtm.connect/test is successful with the same token as used when running code sample above.

Actual result:

  File "C:\Program Files\Python36\lib\asyncio\base_events.py", line 473, in run_until_complete
    return future.result()
  File "D:\Repos\analysis-tools\slack-bot\src\env\lib\site-packages\slack_sdk\rtm\__init__.py", line 360, in _connect_and_read
    proxy=self.proxy,
  File "D:\Repos\analysis-tools\slack-bot\src\env\lib\site-packages\aiohttp\client.py", line 1012, in __aenter__
    self._resp = await self._coro
  File "D:\Repos\analysis-tools\slack-bot\src\env\lib\site-packages\aiohttp\client.py", line 738, in _ws_connect
    headers=resp.headers)
aiohttp.client_exceptions.WSServerHandshakeError: 404, message='Invalid response status', url=URL('wss://cerberus-xxxx.lb.slack-msgs.com/websocket/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX')

[aoberoi]

For those of you experiencing this issue, we’d like to know if you are (👍) or are not (👎) using the proxy option to connect via a proxy. Would you mind reacting to this comment?

[jusdino]

Same problem. Python 3.6, OpenSSL 1.1.1.

Looks like there may be a lot of us. Perhaps we should switch to 👍 on the post if we have no new info to add?