old_contents: 
  def is_siret(siret):
      # A valid SIRET is composed by 14 digits
      try:
          int(siret)
      except ValueError:
          return False
  
      return len(siret) == 14
  
new_contents: 
  def is_siret(siret):
      # A valid SIRET is composed by 14 digits
      return len(siret) == 14 and siret.isdigit()
  
subject: Use isdigit() instead of int()