fix: python/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192442
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14192443

Author: snyk-bot

python/requirements.txt

@@ -7,3 +7,4 @@ construct>=2.9,!=2.10.55
 typing_extensions>=3.10
 dataclasses ; python_version<'3.7'
 simple-rlp>=0.1.2 ; python_version>='3.7'
+urllib3>=2.6.0 # not directly required, pinned by Snyk to avoid a vulnerability