Pychord

- Rename `6/9` to `69`.
- Add `m69`, `m7b5` and `-` qualities.
- Contributor: dok

- Add qualities.
- `sus`, `maj7`, `maj9`, `m6`, `madd9`
- `7b5`, `75`, `7b9`, `79`
- `9-5`, `9b5`, `9+5`, `95`
- `79b5`, `795`, `7b9b5`, `7b95`, `711`, `7b99`, `7b911`, `7911`, `7b13`
- `7b9b13`, `9+11`, `911`, `13-9`, `13b9`, `13+9`, `139`, `13+11`, `1311`
- Support quality alias comparison.
- `maj7 == M7`

- Add Chord.from_note_index method.
- Support chord creation using note index in a scale.
- Contributor: kwadwo00

- Make `QUALITY_DICT` values immutable.

- Raise TypeError in `__eq__` methods.

- Implement `__eq__` method for Quality.
- Fix `__eq__` method of Chord to support comparison between sharped and flatted chords.