Stop raising asyncio.CancelledError if the connection is already closed

[mattvonarx]

Summary

There are now noisy log messages when closing a connection.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.signature (Request Signature Verifier)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.models (UI component builders)
• slack_sdk.scim (SCIM API client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.rtm_v2 (RTM client)
• /docs-src (Documents, have you run ./scripts/docs.sh?)
• /docs-src-v2 (Documents, have you run ./scripts/docs-v2.sh?)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[CLAassistant]

All committers have signed the CLA.

[srajiang]

Hi @VectorAlpha - Thanks for submitting this contribution!
Would you please sign the contributor license agreement?

[seratch]

Hi @VectorAlpha, can I ask you to provide a bit more details of the verbose logging that you've encountered?

[codecov]

Codecov Report

Merging #1152 (73593cb) into main (2dee665) will decrease coverage by 0.06%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1152      +/-   ##
==========================================
- Coverage   86.67%   86.61%   -0.07%     
==========================================
  Files         110      110              
  Lines       10771    10774       +3     
==========================================
- Hits         9336     9332       -4     
- Misses       1435     1442       +7     

Impacted Files	Coverage Δ
slack_sdk/socket_mode/async_client.py	72.50% <66.66%> (-4.43%)	⬇️
slack_sdk/socket_mode/builtin/internals.py	72.00% <0.00%> (-0.45%)	⬇️

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