Log Message:
py-music21: update to 7.3.3

This update doesn't add much for pkgsrc users, given the focus was on
a Python 3.7-only component, but keeps us up to date with code changes.

Change log:

Music21 7.3.3 adds support for showing MIDI inside a Google Colaboratory
(colab) notebook, via music21j. Since m21 v8 will be Python 3.8+ only,
and Google Colab is still on Python 3.7, a back-port to v7 was needed.

7.3.2 was internal development only and not released