Localizer: support python3 and polib 1.2.0

Fix po_import behavior in Python 2.

Newer versions of polib accept only unicode strings in the pofile function (because they check if they start by the decoded version of the BOM). I changed the data that is passed to pofile to be a unicode string in Python 2 too. This seems to work locally in my old version of polib, so that at least the old behavior should be kept the same.

Co-authored-by: Carlos Ramos Carreño carlos.ramos@nexedi.com Co-authored-by: Jérome Perrin jerome@nexedi.com