Maintenance: add Plone 6.1, drop 5.2; add Python 3.12, 3.13, drop 3.9

[stevepiercy]

Should this package be updated to:

1. Add Plone 6.1?
2. Drop 5.2?
3. Add Python 3.12 and 3.13?
4. Drop Python 3.9?

Refs: #448