Cassis 0.9.1

[reckart]

Checklist

• [x] Make sure all issues for the milestone are completed, otherwise move them
• [x] Checkout the main branch
• [x] Bump the version in cassis/__version__.py to a stable one, e.g. __version__ = "0.6.0", commit and push
• [x] Check and wait that the build completed.
• [x] Create a tag for that version and push it
• [x] Wait for the build and Pypi to show the new version
• [x] Bump the version to the next development version, commit and push that.
• [x] Convert the tag to a release on Github, write release note based on issues in the respective milestone
• [x] Create a new milestone