For Py3k, return bytes for non-utf8 C++ strings.

If a wrapped C++ method returns std::string or "const char *", the
wrappers try to decode it as utf-8 in order to return a unicode str()
in Python 3.  Originally, this meant that any decoding error led to
a UnicodeDecodeError exception.  After this change, if decoding utf-8
fails, the C++ string is returned as a raw bytes() object.