add constraint on websockets from sanic

[vcidst]

Proposed changes:

• Sanic 21.12.0 breaks with websockets==11.0, add a constraint to fix it for the time being

Status (please check what you already did):

• [x] made PR ready for code review
• [ ] added some tests for the functionality
• [ ] updated the documentation in the rasaHQ/rasa
• [x] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)

[vcidst]

Add the constraint to pyproject.toml instead of poetry.lock since the latter can be overwritten by automated actions