[REF] requirements.txt: downgrade lxml to 3.5.0
[REF] requirements.txt: downgrade lxml to 3.5.0

In order to circumvent https://bugs.launchpad.net/lxml/+bug/1625506

dfang/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo