add dependabot configuration

[wwuck]

This PR adds a dependabot configuration to keep python dependencies and GitHub actions up to date.

You can read more about this feature at https://docs.github.com/en/code-security/supply-chain-security/keeping-your-dependencies-updated-automatically/about-dependabot-version-updates.

GitHub also provides a feature to get security update PRs via dependabot: https://docs.github.com/en/code-security/supply-chain-security/managing-vulnerabilities-in-your-projects-dependencies/about-managing-vulnerable-dependencies

Continuation of #34

[mbaak]

Thanks!