Update to Python 3.13

[jwhitlock]

Python 3.13 is released. Our pattern has been:

• Add CircleCI jobs to test the new Python
• Update to previous version (Python 3.12, Upgrade to Python 3.12 #3982)
• Wait a bit

In H2 2024, we have a new role of Base Load Engineer, so it may be more natural to fit this into the normal workflow. We'd want to do this before or after the Django 5.2 update, which is planned for release around April 2025. For example, we may update to Python 3.13 in Q1 2025, and Django 5.2 in Q2 2025.