Make default encoding utf8 for unpacking

Is this okay? Please advise.

```
  File ".../xlwt/xlwt/Workbook.py", line 662, in save
    doc.save(filename, self.get_biff_data())
  File ".../xlwt/xlwt/Workbook.py", line 630, in get_biff_data
    before += self.__all_fonts_num_formats_xf_styles_rec()
  File ".../xlwt/xlwt/Workbook.py", line 533, in __all_fonts_num_formats_xf_styles_rec
    return self.__styles.get_biff_data()
  File ".../xlwt/xlwt/Style.py", line 183, in get_biff_data
    result += self._all_num_formats()
  File ".../xlwt/xlwt/Style.py", line 208, in _all_num_formats
    result += NumberFormatRecord(fmtidx, fmtstr).get()
  File ".../xlwt/xlwt/BIFFRecords.py", line 789, in __init__
    ufmtstr = upack2(fmtstr)
  File ".../xlwt/xlwt/UnicodeUtils.py", line 50, in upack2
    us = unicode(s, encoding)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in position 0: ordinal not in range(128)
```