fix RichTextElement.elements items are never promoted to a proper Python object type

[k1e1n04]

Summary

This pull request resolves #1468

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.

[seratch]

Code formatting for the test code is missing. Could you fix it?

[k1e1n04]

I have made the necessary corrections. Could you please check and confirm if everything is in order?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.24%. Comparing base (059dd68) to head (58c92d3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1492      +/-   ##
==========================================
+ Coverage   85.20%   85.24%   +0.04%     
==========================================
  Files         111      111              
  Lines       12291    12291              
==========================================
+ Hits        10472    10477       +5     
+ Misses       1819     1814       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[seratch]

Thank you; LGTM