textproc/py-mkdocstrings: Update to 0.29.1
- Adjust USE=python accordingly, because Python 3.8 is no longer present in the ports tree after ca15850a2b13 while I'm here.
Changelog:
https://github.com/mkdocstrings/mkdocstrings/releases/tag/0.29.1
MFH: 2025Q2