Hotfix: Fix import of Slack API

[TimJentzsch]

Relevant issue: N/A

Description:

This PR attempts to fix a wrong import of the Slack API which resulted in the Blossom deployment failing.

PyCharm warns with "Package containing module 'slack' is not listed in the project requirements" for the import so I'm not 100% sure if this fixes the problem.

Checklist:

• [ ] Code Quality
• [ ] Pep-8
• [ ] Tests (if applicable)
• [ ] Success Criteria Met
• [ ] Inline Documentation
• [ ] Wiki Documentation (if applicable)

[itsthejoker]

guess we'll find out