Use the slack_sdk instead of the deprecated slackclient. Because according to the official documentation:
> The slackclient PyPI project is in maintenance mode now and slack-sdk project is the successor.
With this commit one UPGRADE BLOCKER from requirements/requirements.in is removed. Als the license for slack_sdk
is updated and unit tests for slack notifications backend are added.
Signed-off-by: Daniel Ziegenberg <daniel@ziegenberg.at>