fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JSONPICKLE-8136229

Author: snyk-bot

requirements.txt

@@ -4,3 +4,4 @@ rasa==2.8.21 # NB! when updating, make sure to also update:
 -r actions/requirements-actions.txt
 pytablewriter
 python-dotenv~=0.15.0
+jsonpickle>=3.3.0 # not directly required, pinned by Snyk to avoid a vulnerability