The OPML-Icon, as available on OPML-Icon website.
All credits for these icons go to their original author: Ken Rossi and all the project contributors.
This is the OPML-Icon: 
The aim of this project is to make this icon pack available through various package managers, such as:
npm install opml-icon
bower install opml-icon
composer require legacy-icons/opml-icon
Install-Package opml-icon
See License