feat(dependabot): :sparkles: add dependabot for updating Python dependencies

[martonvago]

Description

This PR adds dependabot and configures it to update Python dependencies (managed by Poetry) in this repository. The PRs will look something like:

See also Issue https://github.com/seedcase-project/.github/issues/24

Testing

• [ ] Yes
• [x] No, not needed (give a reason below)
• [ ] No, I need help writing them

I tested it in a minimal repo and it seemed to behave as expected.

Reviewer Focus

This PR only needs a quick review.

Checklist

For all PRs that are not general documentation

• [x] Tests accompany or reflect changes to the code
• [x] Tests ran and passed locally
• [x] Ran the linter and formatter
• [x] Build has passed locally
• [x] Relevant documentation has been updated

[martonvago]

I read around a bit about what some sensible default settings might be, but it's still pretty bare bones, so feel free to ignore or change.

I'm also not sure if any other future repos might want to use the same setup, in which case we might want it to be synced across them like some other files.