Decide Python versions we want to support

[andrewwuan]

Suggestions are welcome. If not, I'll see if we can support everything that's supported in librosa: https://github.com/librosa/librosa/blob/master/setup.py#L36

[raymondxyy]

Let's not support Python 2.X. There is at least one function I implemented that uses Python 3's built-ins. Also, Python 2 is reaching the end of life anyway.

My preference would be Python 3.6+, but I need to check how the earlier versions affect things. We can support those as well if everything is compatible.