create dependabot config

[2bndy5]

To automate updates for some dependencies, this PR enables github's dependabot for pip requirements.txt files and github actions used in CI workflows.

See GH docs for more detail.

[2bndy5]

This will avoid sporadic warnings from github actions. Currently, We're getting the following:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, actions/setup-node@v3, actions/setup-python@v4, actions/cache@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

Using bot commands

We'll have to tell dependabot to ignore tests/requirements-sphinx4.txt though. Depending on your preference, we could also tell dependabot to ignore some github actions that we pin using commit hashes.