Make pathlib dependency in tests conditional
Make pathlib dependency in tests conditional

We already do this in the optional dependency section so this shouldn't cause any problems. It's only used by the `thumbnails` plugin.
sampsyo/beets,SusannaMaria/beets,sampsyo/beets,jackwilsdon/beets,sampsyo/beets,beetbox/beets,beetbox/beets,beetbox/beets,SusannaMaria/beets,shamangeorge/beets,jackwilsdon/beets,shamangeorge/beets,SusannaMaria/beets,shamangeorge/beets,ibmibmibm/beets,ibmibmibm/beets,ibmibmibm/beets,ibmibmibm/beets,jackwilsdon/beets,sampsyo/beets,jackwilsdon/beets,beetbox/beets,shamangeorge/beets,SusannaMaria/beets