Revert "stack/slapos: version ↑ jsonschema 3.0.0a3"

This reverts commit b3394953.
8 jobs for 1.0
Status Job ID Name Coverage
  External
failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

00:19:34

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

12:56:00

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:13:08

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

07:17:53

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

06:11:50

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

04:14:37

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

03:16:38