Feature: add Python 3.13 support

[jayaddison]

• Confirm locally that the testsuite passes (except for two expected failures) using version 3.13 of Python.
• Confirm that main GitHub Actions continuous integration test workflow passes using the same.
• Update other GitHub Actions workflows to use py3.13
• Update package metadata and internal references to indicate support for py3.13
• Add a changelog entry

Contributor checklist

• Included tests for the changes.
• A change note is created in changelog.d/ (see changelog.d/README.md for instructions) or the PR text says "no changelog needed".

Maintainer checklist

• If no changelog is needed, apply the skip-changelog label.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

Resolves #2138.
Supersedes #2246.