[3.18] community/verovio: upgrade to 3.15 and fix py3-verovio subpackage

Backporting the fix from !50943 (merged)

I think it was upstream PR https://github.com/rism-digital/verovio/pull/3169 which fixed the python packaging (which begs the question if verovio 3.14 ever even worked in python...)

Anyway, I'm going with the smallest version bump that will fix this issue.