Fix a bug on the aiohttp-based Socket Mode client's exception handling

[ifmcnichols]

Summary

__init__.py redeclared type before using it, leading to an error when the exception in receive_messages was reached.

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.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @ifmcnichols to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thanks for fixing this 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (19134d1) 85.44% compared to head (004aed0) 85.48%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1440      +/-   ##
==========================================
+ Coverage   85.44%   85.48%   +0.04%     
==========================================
  Files         111      111              
  Lines       12113    12113              
==========================================
+ Hits        10350    10355       +5     
+ Misses       1763     1758       -5     

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