v3.4.2

Changes

• #971 #972 #973 v3.4.1 fails with serialized attachments parameter in chat.* method calls - Thanks @pmarkert @eddyg

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• All issues/pull requests: https://github.com/slackapi/python-slack-sdk/milestone/34?closed=1
• All changes: https://github.com/slackapi/python-slack-sdk/compare/v3.4.1...v3.4.2