software/erp5/test: fix ResourceWarning on python3

 - use the requests.Session in a context manager so that resources are
properly closed.
 - close the stdout of caucase process we want to keep silent

See merge request !1075
4 jobs for erp5-openidconnect
Status Job ID Name Coverage
  External
passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

14:20:03

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master