Purely from personal experience (I know, I know), Flask's Python 3 support is solid. With a little bit of intuition, I'm sure any Py3-related problems from following the book could be worked around.
Because the Government Digital Service are, for the most part, ideologues. Disagree though I may with much of their policy, it's novel that a Government department actually have drive and direction.
Love their line of F/OSS, but they've done nothing with it, other than Gov.uk. I can't see them accepting pseudonymous pull requests, though.