FWIW this code is important for very old versions of lyx, older than 1.1.5 (released 19 years ago - 2000/06/06).
Funny fact of the day, byte strings do not behave as regular strings in python3 when taking and index.
To get a sub-string we need to pass a range, a integer index will not work as it happens in a regular string:
$ ipython3
...
In [30]: line
Out[30]: b'#This file was created by <mike> Tue Jan 25 10:36:51 2000'
In [31]: line[0]
Out[31]: 35
In [32]: line[0:1]
Out[32]: b'#'
The range notations works for both byte and regular strings in python 3, and it also works in python 2.
Thus the change is simple and effective. In any case I should confess that I was quite surprised by this. :-)
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