[Python-3000] Eliminating PY_SSIZE_T_CLEAN

["Martin v. Löwis"]

I just noticed that the Python 3 C API still contains PY_SSIZE_T_CLEAN.

This macro was a transition mechanism, to allow extensions to use
Py_ssize_t in PyArg_ParseTuple, while allowing other module continue
to use int.

In Python 3, I would like the mechanism, making Py_ssize_t the only
valid data type for size in, say, s# parsers.

Is it ok to still change that?

Regards,
Martin