stack/slapos: restore an old pin of importlib-metadata for py2

1c4a3c69 (stack/slapos.cfg: version up eggs (requests 2.31.0 and
friends), 2023-10-04) broke component/nxdtest/test-py2.cfg and
others using pytest on python2
13 jobs for for_testrunner_1
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:08:38

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:02:45

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:49

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:35

failed SlapOS.SoftwareReleases.IntegrationTest-Master

07:46:36

passed SlapOS.Theia.ResilienceTest-Master-ERP5

08:31:51

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:02:08

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:33

passed SlapOS.Eggs.UnitTest-Master.Python3

00:17:48

passed SlapOS.Eggs.UnitTest-Master.Python3

00:18:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:26:08