version 3.16.0

Changes

• #1207 Add message metadata support - Thanks @seratch
• #1195 Add domain to team.info API parameters - Thanks @seratch
• #1206 Fix #1200 Model to_dict doesn't handle tuple sequences - Thanks @kezabelle @seratch

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• All issues/pull requests: https://github.com/slackapi/python-slack-sdk/milestone/56?closed=1
• All changes: v3.15.2...v3.16.0