For all programs, for the simple reason that:
> Various standard library functionality now wanted unicode str and didn't accept bytes, even though the Python 2 implementation used the equivalent of bytes.
Much of the stdlib works with native strings and will either blow up or misbehave if fed anything else[0], which means much of your codebase will necessarily be native strings, with a subset being explicitly bytes or unicode.
> Repository data is bytes, not Unicode.
It's also mostly absent from the source code, and where it is present (e.g. placeholders or separators) it's easy to flag as explicitly bytes.
[0] though some e.g. the encoding layers or io module want either bytes or unicode depending what you're doing specifically, and not always the most sensible, like baseXY being bytes -> bytes conversions where 95% of the use case is to smuggle binary data through text… oh well
