feat: add Python 3.13 compatibility CI

[rasteja]

Summary

• add an advisory Python 3.13 PSA unit-test job
• make the established Python 3.12 unit-test runtime explicit
• declare Python 3.13 in package classifiers

Validation

• poetry run pytest -q tests/unit --junitxml=test-results/ADDON-89515-python-3.13-unit.xml
• 525 passed on Python 3.13.12

Follow-up

• review the first remote Python 3.13 CI run
• make Python 3.13 blocking in the unit-test matrix after it is stable
• validate the candidate against three representative TAs