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.

product/Localizer/MessageCatalog.py

@@ -660,10 +660,7 @@ class MessageCatalog(LanguageManager, ObjectManager, SimpleItem):
         messages = self._messages
 
         # Load the data
-        if isinstance(data, bytes): # six.PY2
-            pass
-        elif isinstance(data, bytes): # six.PY3
-            data = data.decode()
+        data = data.decode("utf-8")
         po = polib.pofile(data)
         encoding = to_str(po.encoding)
         for entry in po: