py3: officially support Python 3

All tests pass and no known regressions.

docs: Fix a couple of build warnings

Now down to the usual:
build/docs/api/models.rst: WARNING: document isn't included in any toctree

docs: move authentication info to separate file

Goal is to reduce the gigantic 'setup' page in size, and make the
information more understandable. Currently, the different topics do not
really belong together.
This is one small step towards that goal.