slapos-node package: use python3.7 for now

python3.8 can run only buildout versions greater than 2.7.1+slapos019
which restrict a lot the number of SR buildable by slapos.

See merge request !1240
6 jobs for 1.0
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

passed SlapOS.KVM.ResilienceTest-Master

passed SlapOS.SlapRunner.ResilienceTest-Master

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2