Use-composition-not-inheritence for ezproxy

[kousu]

I've falled out of love with inheritence trees and mixins. I would rather make the ezproxy instances I create return a fresh object, with no parent class, that fits the requests.Session API but is not a Session.

I think the easiest way to do this is to override getattribute to proxy calls to requests.Session, except for those we explicitly define.
... but maybe that's just reimplementing python's inheritence lookup rules all over again? I'm not sure. Anyway, it'll be a good excercise to try.