First of all, thank you everyone for your questions and comments. I am thrilled that the prospect of an open source browser-based Python curriculum has attracted so much interest.
I just finished reading all the responses here, on Twitter, and in the Gitter Python room.
I agree with @erictleung, @rmdawson71 and everyone else that we should teach Python 3.
Even though I originally suggested building the challenges here on the forum, it might be easier to start off by building a stand-alone static app on GitHub.io. We did this for the Hour of Code: https://www.freecodecamp.com/hour-of-code/
We could fork this, and each challenge could have a instructions, and an embedded REPL.it where the user can get the tests to pass. Then they could manually report completion, which would be stored in their in their local storage. Then they would be advanced to the next challenge.
It could have its own challenge map and everything.
This way, we wouldn't have to worry about sharing authentication with freeCodeCamp itself or maintaining a database. It would also allow for extremely rapid iterating (new code pushed to the gh-pages branch would instantly be live).
We could take things from there. What do you all think about this lean, iterative approach?