Bump to v.0.12.1

Bump to v.0.12.0

This mainly adjusts 'mutwo.midi' to the new 'mutwo.core >= 2.0.0' release.

Removed
=======
- numpy dependency, see 69d20e5

Bump to version v.0.11.0

Upgrade dependencies and drop python 3.9 support

EventToMidiFile convert return MidiFile and fixes

- `n_midi_channels_per_track` to `midi_channel_count_per_track`
- `EventToMidiFile.convert()` always returns a `mido.MidiFile`
- `EventToMidiFile.convert` can be called without any `path` argument (then it will simply return the `MidiFile` object, but don't write anything to disk).

Expose version to doc

Refactor to mutwo.midi & upgrade mutwo dependencies

Update to new music version

Allow higher mutwo music version

Allow higher mutwo ext music version