Configure dependabot to update dependencies weekly

[legoktm]

Status

Ready for review

Description of Changes

dependabot should submit no more than five pull requests once a week, automatically updating dependencies via pyproject.toml/poetry.lock.

This is somewhat conservative for now as an experiment, we can tweak it if we want faster updates or more of them at once.

Testing

• Visual review, cross-reference with https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy

Release

• No