Log Message:
py-music21: update to 7.3.1

The full upstream change log can't be condensed easily, highlights are:

Version 7.3 is a bug-fix, improvement, and maintenance release of music21. Aside \ 
from a few obscure corners of the code, it is designed to be fully backwards \ 
compatible with the previous v7 releases (7.1). It represents half a year's work \ 
since the original v7.1 release.

Thanks go especially to Jacob Tyler Walls who really steered the contributions \ 
to this release.

Among the major improvements in this branch are:

    Full support for Python 3.10.
        I've been doing all my recent dev on 3.10 taking advantage of new \ 
features, and it's great, and plays well with numpy, tensorflow, and so many \ 
other cutting edge analysis tools.
        In keeping with music21's policy of supporting the last three versions \ 
of Python, music21 v7.3 will be the last version of music21 to support Python \ 
3.7. A decision on whether 3.8 or 3.9 will be the minimum version for music21 v8 \ 
will come later, depending on its release date; we may also update policy \ 
somewhat and raise the minimum version to 3.9 during the v8 cycle.

(7.3.0 had an installation bug -- replacing with 7.3.1)

For full details, please refer to
https://github.com/cuthbertLab/music21/releases/tag/v7.3.0