setup: tweak error message for Python 3 We now have beta support for Python 3. In my opinion, it isn't yet stable enough to allow `pip install Mercurial` to work with Python 3 out of the box: we don't want people accidentally using Mercurial with Python 3 just yet. But I do think we should be more friendly about informing people of their options. This commit tweaks the error message that users see when running setup.py with Python 3. We instruct them about the current level of Python 3 support, point them at the wiki for more info, and give them instructions on how to bypass the check. As part of this, I also changed which version value is printed, as we were printing a named tuple before.