This works for me:
fd = urllib.urlopen(cover)
# Drop the entire PIL part
covr = MP4Cover(fd.read(), getattr(
MP4Cover,
'FORMAT_PNG' if cover.endswith('png') else 'FORMAT_JPEG'
))
fd.close() # always a good thing to do
audio['covr'] = [covr] # make sure it's a list
audio.save()