fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

Author: snyk-bot

requirements.txt

@@ -4,4 +4,5 @@ lifelines==0.27.4
 numba==0.56.4
 numpy-indexed==0.3.5
 reliability==0.8.6
-matplotlib==3.6
+matplotlib==3.6
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability