Anything! Contribution is just about process and coordination.
The most important factor limiting most open source project maintainers is time; anything you can do to minimize the time taken to pull your changes will greatly increase the chance of them being accepted.
Make sure you write unit tests for any changes; this is essential to verify that your changes have been tested to work and under what conditions, and helps cut down time for maintainers to review your code.
I'd suggest searching GitHub for Python projects, find one that sparks your interest, then have a look at any outstanding issues (defects or missing features) and see if there's any low-hanging fruit.