docs: def getAverageBuildDuration(self, package, **kwargs):
          """
          Return a timedelta that Koji considers to be average for this package.
  
          Calls "getAverageBuildDuration" XML-RPC.
  
          :param package: ``str``, for example "ceph"
          :returns: deferred that when fired returns a datetime object for the
                    estimated duration, or None if we could find no estimate for
                    this package.
          """
          seconds = yield self.call('getAverageBuildDuration', package, **kwargs)
          if seconds is None:
              defer.returnValue(None)
          defer.returnValue(timedelta(seconds=seconds))