old_contents: # Copyright 2019 Akretion France <https://akretion.com/>
  # @author: Alexis de Lattre <alexis.delattre@akretion.com>
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
  
  from odoo import _, models
  from odoo.exceptions import UserError
  
  
  class AccountMove(models.Model):
      _inherit = "account.move"
  
      def action_post(self):
          for move in self:
              for line in move.line_ids:
                  if line.product_id and line.product_id.must_have_dates:
                      if not line.start_date or not line.end_date:
                          raise UserError(
                              _(
                                  "Missing Start Date and End Date for invoice "
                                  "line with Product '%s' which has the "
                                  "property 'Must Have Start and End Dates'."
                              )
                              % (line.product_id.display_name)
                          )
          return super(AccountMove, self).action_post()
  
new_contents: # Copyright 2019 Akretion France <https://akretion.com/>
  # @author: Alexis de Lattre <alexis.delattre@akretion.com>
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
  
  from odoo import _, models
  from odoo.exceptions import UserError
  
  
  class AccountMove(models.Model):
      _inherit = "account.move"
  
      def action_post(self):
          for move in self:
              for line in move.line_ids:
                  if line.product_id and line.product_id.must_have_dates:
                      if not line.start_date or not line.end_date:
                          raise UserError(
                              _(
                                  "Missing Start Date and End Date for invoice "
                                  "line with Product '%s' which has the "
                                  "property 'Must Have Start and End Dates'."
                              )
                              % (line.product_id.display_name)
                          )
          return super().action_post()
  
subject: Use super() instead of super(classname, self)