fix: python/requirements-optional.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-optional.txt

@@ -3,3 +3,4 @@ web3 >= 4.8
 Pillow
 stellar-sdk>=4.0.0,<6.0.0
 rlp>=1.1.0 ; python_version<'3.7'
+urllib3>=2.6.0 # not directly required, pinned by Snyk to avoid a vulnerability