music21 v5.7.1

A maintenance release for music21 v5 that fixes two substantial errors:

(1) corpus.search works on Windows as well as Macs

(2) .mxl files created by latest version of Finale or any other system that places .musicxml (not .xml) files within .mxl will be read.

This is just a bug-fix version. More major work is underway on music21 v.6, available on the Github site.

Documentation for this version is identical to v5.7.0