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
3 jobs for fix/erp5-sr-test-resourcewarnings in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

05:57:17

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-WIP

05:13:02

canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-Python3

09:42:33