Test on Python 3.13 and upgrade GitHub Actions

[cclauss]

Description

Types of change

Perhaps update to a more current version of https://github.com/google/sentencepiece/releases

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[cclauss]

Python 3.14 (the π version) was released today.

• https://www.python.org/downloads/release/python-3140/
• https://github.com/actions/python-versions/releases