fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-6150717
- https://snyk.io/vuln/SNYK-PYTHON-JINJA2-6809379
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-3319935
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-3319936

Author: snyk-bot

requirements.txt

@@ -4,3 +4,5 @@ python-http-client>=3.2.1
 six==1.11.0
 starkbank-ecdsa>=2.0.1
 more-itertools==5.0.0
+jinja2>=3.1.4 # not directly required, pinned by Snyk to avoid a vulnerability
+werkzeug>=2.2.3 # not directly required, pinned by Snyk to avoid a vulnerability