Set up scheduled automatic updates

[aphedges]

We currently have dependency versions pinned in requirements-lock.txt and .pre-commit-config.yaml, and we will likely have more files with pinned versions in the future. Therefore, @lichtefeld recommended that we look into automation for this task. Here are some initial tools I've come across:

• Renovate: Seems to be the most powerful tool with support for a large number of different files with a large amount of customizability
• Dependabot: Works for Python dependencies but unfortunately doesn't support pre-commit (Support pre-commit dependabot/dependabot-core#1524)
• pre-commit.ci: Only handles .pre-commit-config.yaml files and doesn't appear to handle additional_dependencies, but it's still better than nothing

We should spend some time looking into the various options available before committing to something.

Derived from @aphedges in #8 (comment)