Use slapos.recipe.build:download-unpacked for Python 3 support

hexagonit.recipe.download-1.7.post4 (Nexedi's patch) is not compatible
with Python 3 (yet?).
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.SlapRunner.ResilienceTest-Master

00:24:36

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:20:47

failed SlapOS.SoftwareReleases.IntegrationTest-Master

07:17:43