slapos: patch PyPDF2

PyPDF2 must not overwrite showwarning because it breaks ERP5. ERP5 already overwrites it properly

https://github.com/mstamy2/PyPDF2/issues/67#issuecomment-34387142
3 jobs for custom_pypdf2 in 0 seconds
Status Job ID Name Coverage
  External
canceled SlapOS.SoftwareReleases.IntegrationTest-Jerome-746

00:09:01

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-746

16:04:58

failed SlapOS.SoftwareReleases.IntegrationTest-Jerome-746

12:17:20