Bhargav Golla writes:

Also, I have observed on the PSF GSoC page that it requires students to submit a patch to the sub-organization.

I like this one:

https://bugs.launchpad.net/mailman/+bug/881320

Requires knowledge of gettext and the Python facilities for dealing with it (IIRC xgettext knows about Python now).

I recommend searching Launchpad for the "mailman3" tag; Mailman 2 bugs are somewhat likely to be harder bugs, and definitely solving Mailman 2 bugs is not as useful for learning Mailman architecture and coding -- not to mention requiring you to check out and find your way around a rather different source tree.

HTH, I gotta get back to work...