Port unity to Python 3

Here's my diff so far. I tried to build this locally but the build segfaulted, so I'll try in a PPA tomorrow.

https://blueprints.launchpad.net/ubuntu/+spec/foundations-x-python3-only

Does the linked branch actually fix the Python 3 bug? I don't see it in the merge proposal diff.

No, the right branch is lp:~unity-team/unity/python3-ready (I did a wrong push in the other before, that's why LP got confused).

This bug was fixed in the package unity - 7.4.0+16.04.20151211-0ubuntu1

---------------
unity (7.4.0+16.04.20151211-0ubuntu1) xenial; urgency=medium

  [ Marco Trevisan (Treviño) ]
  * unity: ensure we use python3 for the launch script and
    python2.7 for autopilot (LP: #1512909)

  [ Andrea Azzarone ]
  * Places(Overlay)VScrollBar: impelement gtk-like overlay scrollbars
    (LP: #1481706)
  * Properly hide shortcut overlay window to avoid unnecessary redraws.

  [ Bryan Quigley ]
  * debian/control: Remove telepathy from recommends

  [ Eleni Maria Stea ]
  * Replaced deprecated calls in tests to get rid of compiler warnings

  [ Marco Trevisan (Treviño) ]
  * ApplicationLauncherIcon: if a new window opens while spreading,
    spread again with proper match (LP: #1517455)
  * ExpoLauncherIcon: add Quicklist to switch workspace and see the
    selected one (LP: #750497)
  * PanelIndicatorEntryView: Activate menus on click, by drawing them as
    active (LP: #1513523)
  * PanelMenuView: ignore leave events when triggered by feeding nux
    manually (LP: #1513496)
  * Places(Overlay)VScrollBar: impelement gtk-like overlay scrollbars
    (LP: #1481706)
  * SwitcherModel: allow to add/remove icons dynamically and update them
    when they require it (LP: #1512349)
  * SwitcherView: Add support to Alt+Key-right-to-tab to close the
    selected app/window
  * Switcher{View,Controller}: close active selection on Alt+q (LP:
    #1468820)
  * TextInput: show warning also when num lock is enabled
  * XKeyboardUtil: add ability to get keycodes from any relative
    position
  * debian/rules: enable unit tests on arm64 again (LP: #1514538)

  [ Sebastien Bacher ]
  * Updated apport hook to include: the compiz and xorg details, not
    only after asking users if the issue is a graphical one (which is
    weird question) the upstart logs for unity/ups/lockscreen some
    gsettings configurations (LP: #1508974)

 -- Marco Trevisan (Treviño) <mail@3v1n0.net> Fri, 11 Dec 2015 12:46:16 +0000