Add support for the admin.users.getExpiration API

There's a new API that we should add support for; admin.users.getExpiration

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.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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首先阅读链接的 admin.users.getExpiration API 文档和贡献指南。然后检查 slack_sdk.web.WebClient 的 sync/async 入口点，以遵循现有的 Web API 方法模式。当两个 WebClient 变体都支持新的 API 时，即表示完成。