'Returns:
text: tags in a human-readable format.
"Human-readable" is used loosely here. The format is intended
to mirror that used for Vorbis or APEv2 output, e.g.
``TIT2=My Title``
However, ID3 frames can have multiple keys:
``POPM=user@example.org=3 128/255``'

def pprint(self):

    frames = sorted((Frame.pprint(s) for s in self.values()))
    return '\n'.join(frames)
