Enhance get_keyword_arguments to support new format in Rf 3.2

[aaltat]

Robot Framework 3.2 will (most likely) add support for returning get_keyword_arguments method call in new format, see more details in robotframework/robotframework#3514. In RF 3.1 it is returned like: ['mandatory', 'optional=default', '**kws'] but in RF 3.2 this would be supported: [('x', 1), ('y', None), ('z', True)]. This would ease the conversion of default types in RF side. Add support for new return type in RF 3.2 but do not loose support for RF 3,1 which does not support new format.