Move to Python 3 instead of Python 2.

[tadhg-ohiggins]

Three better than two / For birds in hand, fowl counted, / And, of course, Python.

The changes to requirements.txt and requirements_dev.txt are the result of running pip-compile in a Python 3 environment rather than Python 2.

[tadhg-ohiggins]

It's probably worth trying this locally as an update to an existing checkout—I had some issues when doing that and am not certain I caught all of them.

[cmc333333]

Also, 100 points for the solid haiku

[tadhg-ohiggins]

Thanks @cmc333333 !