Add SciPy 2026

[jameslamb]

Closes #291

Adds SciPy 2026 (U.S.).

ref: https://www.scipy2026.scipy.org/

Notes for Reviewers

I tested this following the steps in the README.

[jameslamb]

I'm new here, but my interpretation from the README is that it's acceptable for me to self-merge this:

If you are a conference organizer, we can give you direct access to the repository, but it is your responsibility to update your own event!

So going to do that. Please @ me if that's not acceptable or if merging this caused any issues, and thanks for maintaining this!