fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-MAKO-16439021

Author: snyk-bot

requirements.txt

@@ -16,3 +16,4 @@ requests
 SQLAlchemy>=1.4.1
 tornado>=5.1
 traitlets>=4.3.2
+mako>=1.3.12 # not directly required, pinned by Snyk to avoid a vulnerability