chore: support python 3.15

[WilliamBergamin]

Summary

This pull request adds Python 3.15 to the supported versions now that Python 3.15.0 is out.

• Adds 3.15 to the unit test matrix in ci-build.yml, keeping every existing entry (PyPy included).
• Sets LATEST_SUPPORTED_PY to 3.15 in ci-build.yml and in scripts/run_validation.sh, so lint, format, mypy, and coverage upload run on 3.15.
• Adds the Programming Language :: Python :: 3.15 classifier to pyproject.toml.

Testing

• Confirm the new 3.15 unit test job appears and passes in this PR's checks.
• Confirm the lint, typecheck, and database jobs now run on Python 3.15 and pass.
• Ran ./scripts/run_validation.sh on a local Python 3.15.0 venv: passed (ruff, lint, mypy clean; 1035 passed, 5 skipped).

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.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.35%. Comparing base (699e1d1) to head (d9683eb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1987   +/-   ##
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  Coverage   84.35%   84.35%           
=======================================
  Files         118      118           
  Lines       13793    13793           
=======================================
  Hits        11635    11635           
  Misses       2158     2158           

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