fix: dev-requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-3164749
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-5805047
- https://snyk.io/vuln/SNYK-PYTHON-FLASK-5490129
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-5595532
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-3319935
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-3319936

dev-requirements.txt

@@ -4,3 +4,7 @@ pytest-cov>=2.5.1
 pytest-flask>=0.15.1
 flasgger>=0.9.1
 PyYAML>=5.4
+certifi>=2023.7.22 # not directly required, pinned by Snyk to avoid a vulnerability
+flask>=2.2.5 # not directly required, pinned by Snyk to avoid a vulnerability
+requests>=2.31.0 # not directly required, pinned by Snyk to avoid a vulnerability
+werkzeug>=2.2.3 # not directly required, pinned by Snyk to avoid a vulnerability