bzr pack fails with signatures on python 3

[Jelmer Vernooij]

from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918723:

  File "/usr/lib/python3/dist-packages/breezy/bzr/pack_repo.py", line 883, in autopack
    return self._do_autopack()
  File "/usr/lib/python3/dist-packages/breezy/bzr/pack_repo.py", line 924, in _do_autopack
    reload_func=self._restart_autopack)
  File "/usr/lib/python3/dist-packages/breezy/bzr/pack_repo.py", line 943, in _execute_pack_operations
    result = packer.pack()
  File "/usr/lib/python3/dist-packages/breezy/bzr/pack_repo.py", line 711, in pack
    return self._create_pack_from_packs()
  File "/usr/lib/python3/dist-packages/breezy/bzr/knitpack_repo.py", line 914, in _create_pack_from_packs
    new_pack.signature_index)
  File "/usr/lib/python3/dist-packages/breezy/bzr/knitpack_repo.py", line 639, in _copy_nodes
    write_index, pb, output_lines=output_lines)
  File "/usr/lib/python3/dist-packages/breezy/bzr/knitpack_repo.py", line 663, in _do_copy_nodes
    bits = value[1:].split(' ')
TypeError: a bytes-like object is required, not 'str'