old_contents: from .base import Shipper
  
  
  class DHL(Shipper):
      barcode_pattern = r'^\d{10}$'
      shipper = 'DHL'
  
new_contents: from .base import Shipper
  
  
  class DHL(Shipper):
      barcode_pattern = r'^\d{10}$'
      shipper = 'DHL'
  
      @property
      def valid_checksum(self):
          sequence, check_digit = self.tracking_number[:-1], self.tracking_number[-1]
          return int(sequence) % 7 == int(check_digit)
  
subject: Add DHL waybill checksum validation