Force utf-8 decoding when running an external command

Baptiste Beauplatrequested to merge
lyknode/debexpo:gpg-utf8 into live

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')

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