fix: backend/requirements.txt to reduce vulnerabilities (#35)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-SETUPTOOLS-3180412
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

backend/requirements.txt

@@ -31,3 +31,4 @@ setuptools>=65.5.1 # not directly required, pinned by Snyk to avoid a vulnerabil
 urllib3>=1.26.5 # not directly required, pinned by Snyk to avoid a vulnerability
 starlette>=0.25.0 # not directly required, pinned by Snyk to avoid a vulnerability
 cryptography>=41.0.5 # not directly required, pinned by Snyk to avoid a vulnerability
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability