Pytest Compatibility

[mscuthbert]

Music21 is now fully compatible with pytest!

This project actually predates the earliest public releases of pytest and was already far along before pytest became a standard part of the Python ecosystem. Thus, music21 had written its own way of running tests because it had to -- there was nothing else standard out there.

In the past 10+ years, pytest has become the standard, but music21's doctest standard -- assuming that from music21 import * has been run -- used to be a big impediment for using it.

Thankfully there are now ways to make pytest work with how music21 has always written its documentation, so this long-desired partnership can take place!

v10 will retain the music21 test directory as a transition stage to using pytest as the sole source of testing in v11+

[coveralls]

coverage: 93.057% (-0.01%) from 93.067%
when pulling 96dbcde on pytest-compat
into 59cf9ab on master.