fix: pass default ssl=True to aiohttp to avoid deprecation warning

[WilliamBergamin]

Summary

This PR aims to fix a warning that shows up if ssl is None, we should pass True instead

Testing

Unit tests should be sufficient

Category

• 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 (Documents)
• /tutorial (PythOnBoardingBot tutorial)
• tests/integration_tests (Automated tests for this library)

Requirements

• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (3e65b9b) to head (2d85ab1).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1843   +/-   ##
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  Coverage   83.99%   83.99%           
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  Files         117      117           
  Lines       13252    13252           
=======================================
  Hits        11131    11131           
  Misses       2121     2121           

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[zimeg]

@WilliamBergamin LGTM and thanks for keeping connections current 🔏

Would this be related to #1491 perhaps? I'm not so familiar with the nuance of the issue, but this change seems to find good default.

[zimeg]

🗣️ ramble: Here's the deprecation I notice is now gone!

  /home/runner/work/python-slack-sdk/python-slack-sdk/slack_sdk/socket_mode/aiohttp/__init__.py:377: DeprecationWarning: ssl=None is deprecated, please use ssl=True