Copies of the licenses under which this software is distributed are located in the Licenses directory. Greater detail as it relates to contributions or other uses are detailed in the docs.
Original releases (2013-2014) were dual licensed under the GPLv3 and the WTFNMFPLv1. From 2015 onward the software continued to be licensed this way, but also licensed under the three clause BSD license to account for using it as a library.
From August, 2018 onward all the multi-licensing was dropped in favour of theApache 2.0 license as it is both GPL compatible and permissive.
Users may accept any one or combination of these licenses, but contributors (in the unlikely event that there are any) must contribute according to the terms of all of them for code to be accepted (so it can continue to be offered under the same terms).
While there may be a valid argument that the terms of the WTFNMFPLv1 are enough to allow distribution under any other license, it just makes things easier to know that they're all covered explicitly.
Since this project could be used as a module and imported into something else, any contributions offered solely under the terms of the GPL will be rejected since the GPL prevents the use of dynamic linking in non-GPL or non-GPL compliant projects. I believe foad.py has a number of appropriate responses for people who insist on using the GPL or the GPL Affero licenses for library modules.
Making a pull request will be interpreted to mean you are submitting code under the terms of all the licenses under which the project is distributed at that time.