use treq instead of requests

[kalikaneko]

in bitmask we removed the requests dependency some time ago. we recently introduced treq, since it has a nice api and simplifies a lot all the http client code.

I think pixelated could depend on treq instead of requests, it also plays nicely with the rest of the async codebase.

[tuliocasagrande]

Hey @kalikaneko, we use requests marginally on some auxiliary resources, but I agree with you it'd be suitable to move everything to treq.

I'm afraid we won't be able to prioritize this at this moment, though.
On the other hand, if you submit a PR with it, I'll be happy to review it.