Update urllib3 library to 1.25.9 to avoid security vulnerability
Summary
#42 updates the version of the urllib3 library to 1.25.9 to avoid vulnerabilities associated with version 1.24.3.
This also meant upgrading the associated requests library version because the previous version of 2.20.0 explicitly only allowed urllib3 < 1.25.
By upgrading requests to 2.22.0, urllib3@1.25.9 was allowed.