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>