GPLv3. It doesn't require attribution, unless specified by the authors alongside the license. They didn't specify that requirement. Nothing on the Readme, no additional information on the License file, nothing in the code files.

> you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms […] [r]equiring preservation of […] author attributions in that material or in the Appropriate Legal Notices displayed by works containing it