aubio backend

hi there,

Not really a bug, not really a PR: I played around with [aubio](https://aubio.org) and audioread to see how they could work together.

Using aubio's current HEAD and python3, this [simple backend for aubio](https://gist.github.com/piem/7e7a236f8c56a725f3e160e72b2e8669) works well, including on remote streams.

This is vastly inefficient though, since aubio's arrays of floats need to be flattened back to a buffer of shorts, but for some reason I thought I should make some noise about it. :-)

cheers, piem

Start by reading the issue's aubio backend gist and comparing its approach with audioread's existing backends. Check how the Python 3 implementation handles remote streams and float-array conversion; the issue does not define an accepted integration scope or a testable definition of done.

Written by the indexing model from the issue text.