fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PROTOBUF-15090738

Author: snyk-io[bot]

requirements.txt

@@ -23,3 +23,4 @@ tweepy
 typing_extensions
 xgboost
 urllib3>=2.6.0 # not directly required, pinned by Snyk to avoid a vulnerability
+protobuf>=6.33.5 # not directly required, pinned by Snyk to avoid a vulnerability