textproc/py-mkdocs-autorefs: Update to 1.4.2

• Remove the no longer needed workaround for deleting remnants of the Mypy cache because this is fixed with this version.

• Also adjust USES=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/autorefs/releases/tag/1.4.2