Fix typing in Message class

[m-vdb]

Proposed changes:

• Fix typing in Message class. It made my builds fail here: https://github.com/RasaHQ/rasa/pull/6211/checks?check_run_id=890593396

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[dakshvar22]

Thanks for adding the types. Can you enhance the logic to handle the exception case?

[dakshvar22]

Thanks for adding this 💯