Move to Python 3

[nigelmegitt]

Python 2.7 is going out of support, so we need to move to Python 3.

Issues currently:

• ebu_tt_live/utils.py", line 117 uses async as a function parameter label, but in python 3 it is a reserved keyword. Change the label.
• ebu_tt_live/utils.py", line 4 uses Queue but it should be queue - python 3 is case sensitive.
• pyxb generates ebu_tt_live/bindingsbut the _ttm module cannot be imported.
• Python3 has different approaches to filters and maps which create different behaviours.

[nigelmegitt]

#489 fixes the issues but the twisted websocket tests fail to a greater or lesser degree, and it isn't clear why - I suspect something has changed in the twisted or pytest or both, or the way that pytest tests twisted deferreds more generally. But I'm slightly guessing.