fix: samples/client/echo_api/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

samples/client/echo_api/python/requirements.txt

@@ -1,5 +1,5 @@
 python_dateutil >= 2.5.3
 setuptools >= 21.0.0
-urllib3 >= 1.25.3, < 2.1.0
+urllib3>=2.6.0
 pydantic >= 2
 typing-extensions >= 4.7.1