Workflow for automatically updating development dependencies

[alberthli]

This PR introduces an automated workflow to update all development dependencies (that is, those in the dev dependency group in the pyproject.toml) for the pixi lock file.

• Modified from this tutorial from pixi
• Checks weekly but can also be run manually
• Automatically opens a reviewable PR that includes a markdown table showing the diff
• Doesn't open a PR if the diff is empty

[pculbertson]

Really slick. Probably should re-run after the mesh caching PR lands (CI failed due to rate limits) but I like the changes a lot.