Patch PyPDF2

PyPDF2 overwrites `warning.showwarning` and this is already confirmed as a major flaw: https://github.com/mstamy2/PyPDF2/issues/67#issuecomment-34387142

But, this is never applied in PyPDF2.

To fix this issue in ERP5, we will copy the patch to our stack and apply it.

See merge request !746