fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-SETUPTOOLS-3113904

Author: snyk-bot

requirements.txt

@@ -12,3 +12,4 @@ gunicorn>=20.0.4
 psycopg2-binary>=2.8.6
 southwark>=0.7.1
 sqlalchemy==1.3.22
+setuptools>=65.5.1 # not directly required, pinned by Snyk to avoid a vulnerability