Use PyC_ParseUnicodeAsBytesAndSize parser instead of "s" / "z" type
specifier. This relates to #111033, resolving Python exceptions which
causes icons not to load (for e.g.).
Now bytes are also supported as path arguments.
While these warnings point to real errors in the code,
PyMethodDef are an exception where functions with different numbers
of arguments are all cast to the same function type.
