code: def last_modified(self):
          
          mtimes = list(map(os.path.getmtime, self.related_archives))
          return max(mtimes) if mtimes else time.time()
docs: The last modified time of the requirement's source distribution archive(s) (a number).
  
          The value of this property is based on the :attr:`related_archives`
          property. If no related archives are found the current time is
          reported. In the balance between not invalidating cached binary
          distributions enough and invalidating them too frequently, this
          property causes the latter to happen.