v3.19.4

Changes

• #1301 Fix #1297 Building user_auth_blocks with slack_sdk.models class objects for chat.unfurl API call fails - Thanks @seratch @injust

---

• All issues/pull requests: https://github.com/slackapi/python-slack-sdk/milestone/73?closed=1
• All changes: https://github.com/slackapi/python-slack-sdk/compare/v3.19.3...v3.19.4