Add support for get_keyword_source API method for Robot Framework 3.2

[aaltat]

Robot Framework 3.2 will enhance the dynamic library API and add toget_keyword_source API call. See more details in robotframework/robotframework#3507 and robotframework/robotframework#3516 Add support to PythonLibCore for the new API.

[pekkaklarck]

This functionality had problems with decorators on Robot side that should probably be fixed here as well. See my comment here for more details:
robotframework/robotframework#3516 (comment)

[aaltat]

Because of this and forgetting the lambda I am thinking to change the functionality towards what the python returns. But definitely needs fixing.

[pekkaklarck]

Taking lambdas etc. into account isn't too hard:
https://github.com/robotframework/robotframework/blob/master/src/robot/running/handlers.py#L164

[aaltat]

Fixed in #33