tox: Don't use Python 3.8 by default
tox: Don't use Python 3.8 by default

3.8.0 final is not released yet. This default set is meant to be a
reasonable list for quick iteration during development.

shamangeorge/beets,SusannaMaria/beets,sampsyo/beets,jackwilsdon/beets,jackwilsdon/beets,beetbox/beets,ibmibmibm/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,SusannaMaria/beets,shamangeorge/beets,beetbox/beets,sampsyo/beets,ibmibmibm/beets,ibmibmibm/beets,shamangeorge/beets,jackwilsdon/beets,SusannaMaria/beets,SusannaMaria/beets,jackwilsdon/beets