pathlib.walk() only available since 3.12

I added [pathlib.walk()](https://docs.python.org/3/library/pathlib.html#pathlib.Path.walk) to my code and expected vermin to flag the updated code as requiring 3.12 minimum but it did not change from my previous 3.8 minimum.

Version is 1.6.0 installed from Arch AUR, running under Python 3.13.1.