python-gnupg uses latin1 as default encoding because GPG itself uses
latin1 as default.
We should not override this default with getpreferredencoding, because
getpreferredencoding maybe returns something else than what GPG is configured
on that system.

Example: On Windows
GPG is run in default mode with 'latin1'
getpreferredencoding returns 'cp1252'

The approach would be now to default to latin1 as it is GPGs default.
And if the User sets a different ecoding for GPG he has to set it in
Gajim aswell.