nkf.c: Change guess result type from Bytes to Unicode

  In Python3's `nkf.guess`, swap `PyBytes_FromString` to
  `PyUnicode_FromString`.
  So when using guess result to other methods, Unicode type is
  convenient.
Merge pull request #2 from ikegami-yukino/py3_guess_result_type

nkf.c: Change guess result type from Bytes to Unicode