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

@@ -79,7 +79,7 @@ tblib>=1.7.0    # So that the django test runner provides tracebacks
 tlds>=2022042700    # Used to teach bleach about which TLDs currently exist
 tqdm>=4.64.0
 Unidecode>=1.3.4
-urllib3>=1.26,<2
+urllib3>=2.6.3
 weasyprint>=64.1
 xml2rfc>=3.23.0
 xym>=0.6,<1.0