fix: samples/server/petstore/python-fastapi/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/server/petstore/python-fastapi/requirements.txt

@@ -29,7 +29,7 @@ Rx==1.6.1
 starlette==0.40.0
 typing-extensions==4.13.2
 ujson==4.0.2
-urllib3==2.5.0
+urllib3==2.6.0
 uvicorn==0.13.4
 uvloop==0.21.0
 watchgod==0.7