CI: Dependabot

[nphilipp]

Story

As a maintainer of Duffy,
I want that the dependencies used for testing are updated regularly,
so we can benefit from bugfixes and notice potential problems.

Acceptance Criteria

• Dependabot is configured in the repository to check for updated dependencies

Background

poetry install uses the dependency versions pinned in poetry.lock for local environments and CI tests. Dependabot checks for available updates and submits pull requests to update these versions.