method `ConcreteScale.deriveByDegree` not working for `scale.MelodicMinorScale` for some degrees

Start by running the reproduction in the issue: scale.MelodicMinorScale('c').deriveByDegree(6, 'c'), which raises IntervalNetworkException: could not find any edges. The failure is in the interval network that ConcreteScale.deriveByDegree builds, so read the music21 scale module's intervalNetwork code and the melodic minor definition. Done means degrees 6 and 7 return pitches without raising, and the existing scale tests still pass.

music21 version

9.1.0

Problem summary

The method ConcreteScale.deriveByDegree is not working for scale.MelodicMinorScale for some degrees (e.g. 6, and 7).

Steps to reproduce

>>> scale.MelodicMinorScale('c').deriveByDegree(6, 'c')

...
music21.scale.intervalNetwork.IntervalNetworkException: could not find any edges

All other scales and degrees seem to work as expected.

for s in [
    scale.MelodicMinorScale,
    scale.MinorScale,
    scale.MajorScale,
    scale.HarmonicMinorScale,
    scale.OctatonicScale,
    scale.WholeToneScale,
    scale.ChromaticScale,
]:
    sc = s('c')
    print(sc.name)
    print([p.name for p in sc.pitches])
    for i in range(1, len(sc.pitches)):
        try:
            derived1 = sc.deriveByDegree(i, "c")
            pitches = [p.name for p in derived1.pitches]
        except Exception as e:
            pitches = f"Exception: {e}"
        print(i, pitches)

Expected vs. actual behavior

Actual behaviour: see output on C melodic minor for degrees 6 and 7.

C melodic minor
['C', 'D', 'E-', 'F', 'G', 'A', 'B', 'C']
1 ['C', 'D', 'E-', 'F', 'G', 'A', 'B', 'C']
2 ['B-', 'C', 'D-', 'E-', 'F', 'G', 'A', 'B-']
3 ['A', 'B', 'C', 'D', 'E', 'F#', 'G#', 'A']
4 ['G', 'A', 'B-', 'C', 'D', 'E', 'F#', 'G']
5 ['F', 'G', 'A-', 'B-', 'C', 'D', 'E', 'F']
6 Exception: could not find any edges
7 Exception: could not find any edges
C minor
['C', 'D', 'E-', 'F', 'G', 'A-', 'B-', 'C']
1 ['C', 'D', 'E-', 'F', 'G', 'A-', 'B-', 'C']
2 ['B-', 'C', 'D-', 'E-', 'F', 'G-', 'A-', 'B-']
3 ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'A']
4 ['G', 'A', 'B-', 'C', 'D', 'E-', 'F', 'G']
5 ['F', 'G', 'A-', 'B-', 'C', 'D-', 'E-', 'F']
6 ['E', 'F#', 'G', 'A', 'B', 'C', 'D', 'E']
7 ['D', 'E', 'F', 'G', 'A', 'B-', 'C', 'D']
C major
['C', 'D', 'E', 'F', 'G', 'A', 'B', 'C']
1 ['C', 'D', 'E', 'F', 'G', 'A', 'B', 'C']
2 ['B-', 'C', 'D', 'E-', 'F', 'G', 'A', 'B-']
3 ['A-', 'B-', 'C', 'D-', 'E-', 'F', 'G', 'A-']
4 ['G', 'A', 'B', 'C', 'D', 'E', 'F#', 'G']
5 ['F', 'G', 'A', 'B-', 'C', 'D', 'E', 'F']
6 ['E-', 'F', 'G', 'A-', 'B-', 'C', 'D', 'E-']
7 ['D-', 'E-', 'F', 'G-', 'A-', 'B-', 'C', 'D-']
C harmonic minor
['C', 'D', 'E-', 'F', 'G', 'A-', 'B', 'C']
1 ['C', 'D', 'E-', 'F', 'G', 'A-', 'B', 'C']
2 ['B-', 'C', 'D-', 'E-', 'F', 'G-', 'A', 'B-']
3 ['A', 'B', 'C', 'D', 'E', 'F', 'G#', 'A']
4 ['G', 'A', 'B-', 'C', 'D', 'E-', 'F#', 'G']
5 ['F', 'G', 'A-', 'B-', 'C', 'D-', 'E', 'F']
6 ['E', 'F#', 'G', 'A', 'B', 'C', 'D#', 'E']
7 ['D-', 'E-', 'F-', 'G-', 'A-', 'B--', 'C', 'D-']
C Octatonic
['C', 'D', 'E-', 'F', 'G-', 'A-', 'A', 'B', 'C']
1 ['C', 'D', 'E-', 'F', 'G-', 'A-', 'A', 'B', 'C']
2 ['B-', 'C', 'D-', 'E-', 'F-', 'G-', 'G', 'A', 'B-']
3 ['A', 'B', 'C', 'D', 'E-', 'F', 'G-', 'A-', 'A']
4 ['G', 'A', 'B-', 'C', 'D-', 'E-', 'F-', 'G-', 'G']
5 ['G-', 'A-', 'A', 'B', 'C', 'D', 'E-', 'F', 'G-']
6 ['F-', 'G-', 'G', 'A', 'B-', 'C', 'D-', 'E-', 'F-']
7 ['E-', 'F', 'G-', 'A-', 'A', 'B', 'C', 'D', 'E-']
8 ['D-', 'E-', 'F-', 'G-', 'G', 'A', 'B-', 'C', 'D-']
C Whole tone
['C', 'D', 'E', 'F#', 'G#', 'A#', 'B#']
1 ['C', 'D', 'E', 'F#', 'G#', 'A#', 'B#']
2 ['B-', 'C', 'D', 'E', 'F#', 'G#', 'A#']
3 ['A-', 'B-', 'C', 'D', 'E', 'F#', 'G#']
4 ['G-', 'A-', 'B-', 'C', 'D', 'E', 'F#']
5 ['F-', 'G-', 'A-', 'B-', 'C', 'D', 'E']
6 ['D', 'E', 'F#', 'G#', 'A#', 'B#', 'D']
C Chromatic
['C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C']
1 ['C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C']
2 ['B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B']
3 ['B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-']
4 ['A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A']
5 ['G#', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-']
6 ['G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#', 'G']
7 ['F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F', 'F#']
8 ['F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E', 'F']
9 ['E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-', 'E']
10 ['E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D', 'E-']
11 ['D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#', 'D']
12 ['C#', 'D', 'E-', 'E', 'F', 'F#', 'G', 'A-', 'A', 'B-', 'B', 'C', 'C#']