PdfFileReader: avoid to overwrite warnings

We already overwrite it in ERP5Type/Utils.py, then _showwarning method from PyPDF2 breaks ERP5

PyPDF2 can not overwrite _showwarning, because we already did it.

I tried to extend test.erp5.testERP5PDFMerge to reproduce this issue, but I can not catch it in unit test. Any idea?

/cc @romain, @vpelletier