fix: pip-sample/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250

pip-sample/requirements.txt

@@ -57,7 +57,7 @@ testpath==0.4.1
 tornado==5.1.1
 traitlets==4.3.2
 tweepy==3.6.0
-urllib3==1.23
+urllib3==1.26.19
 virtualenv==16.0.0
 wcwidth==0.1.7
 webencodings==0.5.1