Make the lyx2lyx code compatible between python2 and python3.

Make the lyx2lyx code compatible between python2 and python3.

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. :-)