Fix another type code bytes 'y' -> string 's' under Python 2.

Should fix this error under Python 2.6,

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ERROR: test_filter_properties_roundtrip (__main__.MiscellaneousTestCase)
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Traceback (most recent call last):
  File test_lzma.py, line 1150, in test_filter_properties_roundtrip
    lzma.FILTER_LZMA1, b]x00x00x80x00)
TypeError: _decode_filter_properties() argument 2 must be impossible<bad format char>, not str
Fix fatal error on Python 2 calling PyString_InternInPlace on NULL