[FIX] mass_mailing: restore py import (prevented to unsubscribe)
[FIX] mass_mailing: restore py import (prevented to unsubscribe)

For unknown reasons, commit bbca13541053a3ac35ac2ec340b843205117468a removed the
import of "_" which was still used. This commit restores it, allowing
users to unsubscribe from mailing without crash.

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