[FIX] l10n_it_edi: Vendor bill is not being created from PEC

When one or more vendor bills in Italian e-invoice format were sent to the registered PEC mailbox, if they are in the PKCS#7 format ('.xml.p7m'), nothing happened.
The e-invoice content should be automatically decoded and the invoice should be created as soon as the xml file is fetched from the PEC mailbox.

A new remove_signature function has been added in a tools/remove_signature.py file to handle this case.
The OpenSSL and pyOpenSSL libraries must be installed.
They are not in requirements.txt but they're probably available.

New tests has been provided for the signed email reading method.

opw-2460485

closes odoo/odoo#71138

Signed-off-by: Josse Colpaert <jco@openerp.com>