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.

Tests for testLocalizer pass in both Python 2 and Python 3.