How to transpose ChordSymbols along with notes in Music21? - music21

Question

I'm trying to transpose a jazz lead sheet. When I transpose the Music21 stream, only the notes are being transposed. The ChordSymbols stay the same.
Here's an example in tinynotation of what I'm seeing. Note: I'm not using TinyNotation in my actual use-case, but it works for providing an example here.
from music21 import *
class HarmonyModifier(tinyNotation.Modifier):
def postParse(self, n):
cs = harmony.ChordSymbol(n.pitch.name + self.modifierData)
self.parent().stream.append(cs)
return n
tnc = tinyNotation.Converter()
tnc.modifierUnderscore = HarmonyModifier
tnc.load("4/4 d2_m7 g4_7 c_maj7")
s = tnc.parse().stream
s.show("text")
s.transpose(interval.GenericInterval(2), inPlace=True)
s.show("text")
And here's the output.
{0.0} <music21.stream.Measure 1 offset=0.0>
{0.0} <music21.clef.BassClef>
{0.0} <music21.meter.TimeSignature 4/4>
{0.0} <music21.harmony.ChordSymbol Dm7>
{0.0} <music21.note.Note D>
{2.0} <music21.harmony.ChordSymbol G7>
{2.0} <music21.note.Note G>
{3.0} <music21.harmony.ChordSymbol Cmaj7>
{3.0} <music21.note.Note C>
{4.0} <music21.bar.Barline style=final>
# Note ChordSymbols are not changed after transpose()
{0.0} <music21.stream.Measure 1 offset=0.0>
{0.0} <music21.clef.BassClef>
{0.0} <music21.meter.TimeSignature 4/4>
{0.0} <music21.harmony.ChordSymbol Dm7>
{0.0} <music21.note.Note E>
{2.0} <music21.harmony.ChordSymbol G7>
{2.0} <music21.note.Note A>
{3.0} <music21.harmony.ChordSymbol Cmaj7>
{3.0} <music21.note.Note D>
{4.0} <music21.bar.Barline style=final>
I'm using music21-4.1.0.