TypeError: %d format: a number is required, not str [Python 3.x + GStreamer 1.0 branch]

When using the Python 3.x + GStreamer 1.0 branch, soundconverter fails with the following exception:

Traceback (most recent call last):
  File "/usr/share/soundconverter/python/soundconverter/gstreamer.py", line 746, in on_task_finished
    newname = self.window.prefs.generate_filename(task.sound_file)
  File "/usr/share/soundconverter/python/soundconverter/ui.py", line 749, in generate_filename
    return generator.get_target_name(sound_file)
  File "/usr/share/soundconverter/python/soundconverter/namegenerator.py", line 94, in get_target_name
    result = pattern % d
TypeError: %d format: a number is required, not str

This bug has been reported as Debian bug #804454 (https://bugs.debian.org/804454)