precommit: use python3 explicitly

This was using `python` which is not always available

Also re-export package-lock.json for the format from npm we currently
have in theia
8 jobs for master in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

00:07:12

passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:31:11

failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

15:02:15

passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

00:31:00

passed SlapOS.Eggs.UnitTest-Master.Python2

00:16:33

canceled SlapOS.SoftwareReleases.IntegrationTest-Master

19:26:22

failed SlapOS.Theia.ResilienceTest-Master-ERP5

08:54:54

passed SlapOS.Eggs.UnitTest-Master.Python2

00:18:23