Fix audio thumbnailing once and for all.

Fix audio thumbnailing once and for all.

This change adds a Python 3-specific audio thumbnailer that side-steps the
bundled Python 2-only `audioprocessing` module. Instead of an audio spectrogram,
Python 3 users will get a static image.

This also allows me to remove my ineffective customisations to
`audioprocessing`, returning it to the upstream version as should always be the
case for vendored code in "extlib".