HN user
To follow up on this, if you've got a spare hour, one of the python core developers has a great demo of how pep8 can sometimes be blinding to larger issues https://www.youtube.com/watch?v=wf-BqAjZb8M
That said, most of what pep8 says is great to follow in the vast majority of cases.
One idea would be to pick a project that you actually use. This means you'll already have an understanding of what the project is about, and you can directly benefit from whatever work you end up doing, which is a nice added incentive. Even big projects will likely have lots of small fixup/documentation issues which no one has gotten around to, but will add value, so don't feel too put off by the size of a project.