[FIX] attribute-string-redundant: "string" of related field is taken from the referenced one

[moylop260]

So redefining the string is a valid case. For instance:

• parent_date = fields.Date(related="parent_id.date", string="parent date")

If a string is not specified, field will be "Date". But I need the string to be "Parent Date"

Fix https://github.com/OCA/pylint-odoo/issues/332