2.21.x convert-ly ?

[Stanton Sanderson]

On Mac OS 10.14, using either Frescobaldi 3.1.2 or directly with the Lilypond 
app (32-bit versions):

when attempting to convert from v2.19.x to 2.21.x 

 \version "2.19.83"
 \relative {
   c' d e f g a b c
 }

the following error message is generated:
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Traceback (most recent call last):
  File "/Applications/LilyPond_2.21.4..app/Contents/Resources/bin/convert-ly", 
line 86, in <module>
    import convertrules
  File 
"/Applications/LilyPond_2.21.4..app/Contents/Resources/share/lilypond/current/python/convertrules.py",
 line 26, in <module>
    import lilylib
  File 
"/Applications/LilyPond_2.21.4..app/Contents/Resources/share/lilypond/current/python/lilylib.py",
 line 30, in <module>
    sys.stdin = codecs.getreader('utf8')(sys.stdin.detach())
AttributeError: 'file' object has no attribute 'detach'


The document has not been changed.
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Sorry if this is an old question or known issue- I did look for an answer.