textproc/py-mkdocstrings-python: Update to 1.16.10

• Adjust USE=python accordingly, because Python 3.8 is no longer present in the ports tree after ca15850a2b13 while I'm here.

Changelog since 1.6.8:

https://github.com/mkdocstrings/python/blob/1.16.10/CHANGELOG.md

MFH: 2025Q2
(cherry picked from commit f24c8a9efd52623d607af515a474b1da67f71753)