fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390193
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390194

Author: snyk-bot

requirements.txt

@@ -12,3 +12,4 @@ owslib
 python-dateutil
 requests
 werkzeug
+urllib3>=2.5.0 # not directly required, pinned by Snyk to avoid a vulnerability