fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-GUNICORN-7856105

requirements.txt

@@ -1,7 +1,7 @@
 fastapi==0.94.1
 httpx==0.23.3
 uvicorn==0.21.0
-gunicorn==20.1.0
+gunicorn==21.2.0
 requests==2.31.0
 pytest==7.2.2
 pyflakes==3.0.1