Set up custom GitHub Codespaces dev container config

[JoshKarpel]

Might be convenient for contributors, plus I'd like to experiment with this workflow.

• https://docs.github.com/en/codespaces/overview
• https://docs.github.com/en/codespaces/setting-up-your-project-for-codespaces/setting-up-your-python-project-for-codespaces

Make sure to update the contributing guide in the docs to note that this is available.