code: def check_extras(dist, attr, value):
      
      try:
          list(itertools.starmap(_check_extra, value.items()))
      except (TypeError, ValueError, AttributeError):
          raise DistutilsSetupError(
              "'extras_require' must be a dictionary whose values are "
              "strings or lists of strings containing valid project/version "
              "requirement specifiers."
          )
docs: Verify that extras_require mapping is valid