conf.py: Update copyright date by StanFromIreland · Pull Request #1497 · python/devguide

Current:

2011 Python Software Foundation

Proposed:

2011-2025 Python Software Foundation

The content of most of the pages has been updated since 2011. This is consistent with the format of the docs.python.org footer:

---

📚 Documentation preview 📚: https://cpython-devguide--1497.org.readthedocs.build/