Using splitByQuarterLengths on stream causes an AttributeError

[ggsimao]

music21 version

9.1.0

Problem summary
Using splitByQuarterLengths on a stream object causes the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/acerola/.local/lib/python3.10/site-packages/music21/base.py", line 3364, in splitByQuarterLengths
    spannerList.extend(st.spannerList)
AttributeError: 'tuple' object has no attribute 'spannerList'

Steps to reproduce

>>> from music21 import stream, note
>>> stream.Measure([note.Note(quarterLength=8)]).splitByQuarterLengths([4, 4])

Expected vs. actual behavior
Expected: the function finishes executing and the stream is properly split.
Actual: AttributeError

More information
The st variable inside the splitAtQuarterLength function is of type _SplitTuple, but when it is returned to the splitByQuarterLengths function it reverts back to a regular tuple.