music21 v6.7.1
Maintenance release for v6.7 --
fixes bugs in lyric.search for working with "composite" lyrics, in the export of piano scores with key signatures, and a bug in piano scores and other part staffs that (a) prevented them from being flattened properly and (b) did not allow flattened piano scores to call `.show()
Other changes and improvements are going into the v7 / main branch.