convert-ly on Mac OS

[Stan Sanderson]

Mac OS 10,14.6, LilyPond 2.21.5-1 (32-bit version, as provided by the LilyPond 
web-site)
Problem: Selecting “Update Syntax” from the “Compile” menu when the default 
test file is loaded returns the following error:

 Traceback (most recent call last):
  File 
"/Users/ssanders/Desktop/LilyPond.app/Contents/Resources/bin/convert-ly", line 
81, in <module>
    import convertrules
  File 
"/Users/ssanders/Desktop/LilyPond.app/Contents/Resources/share/lilypond/current/python/convertrules.py",
 line 27, in <module>
    import lilylib
  File 
"/Users/ssanders/Desktop/LilyPond.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 Python version shown when App 
Contents->Frameworks->Python.framework->Versions are shown is 2.6. 

Thanks,

Stan