I noticed this issue while porting compreffor to py3. In my test fonts, the binary
CFF tables as generated with python 2 sometimes were slightly different from the
ones generated with python 3, although the TTX dump was identical!
It turns out, when running in Python 3, cffLib adds extra entries to the
list of CFF indexed strings, because of bytes vs str.
The `IndexedStrings.getSID` method takes an input string 's' and and returns
the SID integer for that string. If it's a new string, it gets appended to the
list, as well as to an internal strings-to-SID mapping, so that the same SID
value is returned for any given string.
The problem with python 3 was that, if the input string was of `bytes` type
instead of `str`, then the test for inclusion (the dict's `__contains__`)
would return False, and as a result the "same" string (e.g. "Regular" and
b"Regular") could be encoded twice in the list of CFF strings.
(yes, we desperately need unit tests for cffLib...)
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