Hi Kostas,
right now, we're coding challenger against what exists in debian wheezy, which means version 0.1.2 of the requests lib using the python-requests package you mentioned, where response.json is correct, and not response.json() to get json content from the response.
I'd recommend that if you want to make your own "grab stuff from onionoo" script suite, to work with onion-py[1] . It's very new, very spiffy and uses python 3 and the newest requests lib. (full disclosure: It's my baby and I'm desperately looking for testers/users, but that should be obvious to anyone who read this thread.) Alternatively, convince the right people (presumably Karsten and arma) that challenger should switch to a more sustainable runtime than "what we can get from wheezy's repositories". ;-)
Cheers, Luke