[otc2otf] concat str to bytes on Python 3

[mrhso]

On Python 3, otc2otf fails with this message:

D:\体包工作室>otc2otf msyh.ttc
 font: msyh.ttc
Traceback (most recent call last):
  File "c:\program files\python37\lib\runpy.py", line 193, in _run_module_as_mai
n
    "__main__", mod_spec)
  File "c:\program files\python37\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Program Files\Python37\Scripts\otc2otf.exe\__main__.py", line 9, in <
module>
  File "c:\program files\python37\lib\site-packages\afdko\otc2otf.py", line 284,
 in main
    run(sys.argv[1:])
  File "c:\program files\python37\lib\site-packages\afdko\otc2otf.py", line 270,
 in run
    fontEntry = readFontFile(offset, data, tableDict, doReportOnly)
  File "c:\program files\python37\lib\site-packages\afdko\otc2otf.py", line 203,
 in readFontFile
    fontEntry.fileName = fontEntry.psName + ".ttf"
TypeError: can't concat str to bytes