Summary

This is a simple somewhat complicated change that improves type handling when calling .get() on API responses.

For example, this line:

    if api_response and api_response.get("ok") and len(api_response.get("messages", [])):

currently generates this warning in Pyright:

Argument of type "Unknown | None" cannot be assigned to parameter "__obj" of type "Sized" in function "len"
  Type "Unknown | None" cannot be assigned to type "Sized"
    "__len__" is not present

This simple typing change alleviates this issue.

This is the first use of TypeVar in slack_sdk that I see... are there compatibility issues with older Python versions, perhaps? Opening this PR so this fix can at least be considered.

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.