Changelog contents should be UTF-8 text files. There's no need to be
binary-safe. So switch to using text strings in Python (str, not bytes). This
commit makes the following changes:
* Bytes literals (b'…') to string literals ('…').
* Subprocess output (which is all git information) is decoded as ascii.
* Inject text directly in exceptions rather than calling a decode method.
This is enough to make the script work as desired in a UTF-8 locale.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>