Make this index unique, optionally dropping duplicate entries.

            :param drop_dups: Drop duplicate objects while creating the unique \
                index?  Default to ``False``

def unique(self, drop_dups=False):
        ''' Make this index unique, optionally dropping duplicate entries.

            :param drop_dups: Drop duplicate objects while creating the unique \
                index?  Default to ``False``
        '''
        self.__unique = True
        if drop_dups and pymongo.version_tuple >= (2, 7, 5): # pragma: nocover
            raise BadIndexException("drop_dups is not supported on pymongo >= 2.7.5")
        self.__drop_dups = drop_dups
        return self