fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14896210

Author: snyk-bot

requirements.txt

@@ -8,3 +8,4 @@ psutil==5.6.6
 scipy==1.0.0
 scikit-learn==0.19.1
 jinja2==2.10
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability