Port to Python 3 - use rb for message catalog file

Reported and patch proposed by Bas Hulsken.

Fixes https://github.com/sugarlabs/sugar-toolkit-gtk3/issues/425

Signed-off-by: James Cameron <quozl@laptop.org>

src/sugar3/activity/bundlebuilder.py

@@ -155,7 +155,7 @@ class Builder(object):
                 if self._no_fail:
                     continue
 
-            cat = gettext.GNUTranslations(open(mo_file, 'r'))
+            cat = gettext.GNUTranslations(open(mo_file, 'rb'))
             translated_name = cat.gettext(self.config.activity_name)
             translated_summary = cat.gettext(self.config.summary)
             if translated_summary is None: