Python 2 vs Python 3

Hi,

Currently Kresus makes use of Weboob and Python 2. However, Weboob (devel branch) is available on Python 3 now (except for some browser1 modules, but I don't think that concerns any banking module). Moreover, Python 3 will likely be the default setup for upcoming releases.

What about letting the user choose between Python 2 and Python 3, with an environment variable. Typically, add something like WEBOOB_USE_PYTHON_3 (I don't really like this name, but I don't have better ideas right now) environment variable to switch to Python 3?

I can write a MR to handle this change if you think this is interested and you agree with it :)