fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-NLTK-7411380

Author: snyk-bot

requirements.txt

@@ -30,3 +30,4 @@ pyshorteners
 fonttools>=4.43.0 # not directly required, pinned by Snyk to avoid a vulnerability
 holidays>=0.45 # not directly required, pinned by Snyk to avoid a vulnerability
 zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability
+nltk>=3.8.2 # not directly required, pinned by Snyk to avoid a vulnerability