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>