feat(i18n): consider utf-8 to be the encoding used for the python files
When there is no #coding: utf-8
information at the beginning of a file, the
cubicweb-ctl i18ncube
command fails.
As of https://www.python.org/dev/peps/pep-3120/ this information is not needed anymore, and the file should be assumed to be utf-8 encoded.