component/serf: explicitly use python2

This version of scons only support python2
11 jobs for 1.0 in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:57

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:53

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:29:47

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:54

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:05

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:20

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:06

passed SlapOS.SlapRunner.ResilienceTest-Master

00:22:32

failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

01:30:01