gh-35289: pplpy*: update upstream info and deps
### 📚 pplpy/pplpy_doc: update upstream info and deps
these are now hosted on sagemath;
also, build system of pplpy errors out if there is no sphinx.
### 📝 Checklist
- [x] I have made sure that the title is self-explanatory and the
description concisely explains the PR.
URL: https://github.com/sagemath/sage/pull/35289
Reported by: Dima Pasechnik
Reviewer(s): Matthias Köppe
