Revert "Travis: re-enable Python 3.8"
Revert "Travis: re-enable Python 3.8"

This reverts commit 0d44d31913edef1cddaac5461bc33a2c975ec01d.

Apparently, 0.15.5 (which contains the 3.8 fix) is not actually released
yet. Oops!

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