update-hash: use python2

This currently only works with python2, so let's explictly use python2
for now.
2 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
canceled SlapOS.SlapRunner.ResilienceTest-Master

61:11:04

canceled SlapOS.SlapRunner.ResilienceTest-Master.ERP5

61:15:36