Added encoding='utf-8' to debexpo_exec.
This prevent non-UTF-8 locales (thanks apache and mod_wsgi...) to treat output from external commands executed with check_output as anything else as UTF-8.
I've added the text=True argument as a default to debexpo_exec and removed it from all calls.
Added a regression test using setlocale(LC_CTYPE, 'C')