Maybe something like pygame-extras-vX.X (including TMX support and other possible interesting features that other high-level libraries have) that matches exactly (and is totally compatible) the pygame-vX.X would be fine.
This way people that only wants the "core libraries" could just install pygame. And people that wants the "extras" can install the other package among with the core.
That would be different from packing it as a "pygame 3rd party library" (like those that you can download from
pygame.org) because it will appear in the main download page and they will be packages in the APT/YUM repositories of the distros, meaning that they are the "default libraries" for diverse tasks (TMX maps, scenes, X feature, Y support, Z, etc)...
Best regards!