Crash output txt option in python3

[yasuhiroki]

python3 で --output txt オプションを指定するとクラッシュすることがある。

$ export PYTHONPATH=.
$ python3 ./bin/aozora books --output txt
Traceback (most recent call last):
  File "./bin/aozora", line 11, in <module>
    sys.exit(main())
  File "./bin/aozora", line 8, in main
    return aozoracli.cli()
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/cli.py", line 40, in books
    _print(res, output)
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/cli.py", line 98, in _print
    output = aozoracli.output.txt.dump(res)
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 5, in dump
    return _format_print_txt(res)
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 9, in _format_print_txt
    return "\n".join([_to_txt(d) for d in data])
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 9, in <listcomp>
    return "\n".join([_to_txt(d) for d in data])
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 28, in _to_txt
    val = _to_txt(data[key])
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 22, in _to_txt
    output += _to_txt(d)
  File "/Users/yasuhiroki/ghq/github.com/aozorahack/aozora-cli/aozoracli/output/txt.py", line 30, in _to_txt
    return " ".join(sorted_values)
TypeError: sequence item 0: expected str instance, bytes found